Siman 109 . The law of how a person should conduct himself so that his silent Amidah coincides with that of the congregation.

109:1. (1) One who comes into Shul and finds (2) the congregation saying the silent Amidah, if he is able to start and finish [his own Amidah] before (3) the Leader [Shaliach Tzibur] reaches Kedushah (4) or Kaddish he should say the Amidah. {(5) And the same applies to "Amen" to "HoKel HaKodosh" [the conclusion of Kedushah] and to "Shome'ah Tefillah" [the conclusion of the "Shema Koleinu" Blessing] in that these are treated the same way as Kaddish and Kedushah [that he may not start his own Amidah if he thinks he will not be able to say them with the congregation] (Tosefos, the Rosh, the Mordechai, and the Terumas Hadeshen chapter 11)}. But if he is not [able to say his Amidah before the Leader reaches Kedushah, etc.] then he should not say it, provided the time [for saying the Amidah] is not about to pass. If he comes [into Shul] (6) after Kedushah, then if he is able to start and finish [his silent Amidah] before the (7) Leader reaches "Modim" ["We thank"] he should say the Amidah, but if not he should not say it. The same applies if he is able to reach "Modim" or one of the Blessings (8) in which one bows at the same time that the Leader reaches "Modim", then one may say the Amidah. But if he needs to start [his Amidah right away] (9) in order to have it come immediately after [the Blessing of] "Go'al Yisroel" [ie. Someich Ge'ulah LiTefillah see Halacha Yomi on Siman 58 for more details] and it turns out that the Leader will reach "Modim" when he is in the middle of one of the Blessings (10) then he should bow with [the Leader at "Modim"], but if he is at the beginning or end of a Blessing he should not bow [with the Leader] as one may not bow at the beginning or end of a Blessing with the exception of "Avos" [ie. at the beginning and end of the first Blessing] and "HoDo'oh" [the beginning and end of the blessing "Modim...U'l'cha Noeh L'hodos"].

MB 1: One who comes into Shul - If he wants to go out and pray in the foyer outside the Shul, he may do so and need not wait.

MB 2: The congregation saying the silent Amidah etc. - This paragraph [of Shulchan Aruch] is talking about Minchah [the afternoon Service which, unlike Ma'ariv, has a repetition of the Amidah by the Leader]. The same laws also apply to Shacharis [morning Service] in that if he was saying the Shema or its Blessings when the congregation started the silent Amidah and if he was able to start and finish [his own silent] Amidah (up to "Elokai Netzor" - "My G-d guard") [having of course first completed saying the Shema and all its Blessings through Go'al Yisroel] before the Leader reaches Kedushah [in the repetition], then he may go ahead and pray his Amidah. But if not, then he may not pray the Amidah, rather he should wait at "Shiroh Chadoshoh" ["A new song" - the beginning of the final paragraph before the Amidah itself - until the Leader has reached Kedushah so that he may respond to it before continuing --SP]. And see what is written later on this subject in the name of the Chayei Odom.

MB 3: The Leader reaches Kedushah - Since it is forbidden to say Kedushah and also Kaddish on one's own [and so one must wait to say them with the congregation]. As far as "Modim" is concerned, [the Shulchan Aruch's] reason [for requiring one to wait] is that one needs to bow with the congregation [when they reach "Modim"] so that it will not appear that [by not bowing] one is denying [the the existence of] the One to Whom the congregation is bowing.

MB 4: Or Kaddish - Apparently, since one is required to finish one's silent Amidah before Kedushah, all the more so will one hear the Kaddish after the Amidah [so why does the Shulchan Aruch mention Kaddish if, by finishing before Kedushah, one will hear it anyway? --SP], so one might say that [the Shulchan Aruch] is talking about Ma'ariv [Evening Service] when there is no Kedushah [as there is no repetition of the Amidah] or that he is talking about where he has already heard Kedushah, so he will not have to wait [to start his own Amidah] because of [having to hear] Kedushah as we shall write below, but nevertheless he has to finish [his Amidah] before the Kaddish said after the Amidah, ie. he should in any event finish before the Leader reaches "Yehei Shemei Rabboh" [the most important verse in the Kaddish which everyone must say together with great concentration --SP], but he should not say "Amen" at that point, just "Yehei Shemei Rabboh", because he has not been concentrating at all on the paragraph on which "Amen" has been said [ie. the first paragraph of Kaddish. The word "Amen" is used as an affirmation by the listener of what he has heard, and since he was at that point still concentrating on his Amidah, he should not say "Amen"] and it will be considered to be an "Orphaned Amen". But if he directs his heart and knows at that point to what he is answering "Amen", then he is permitted [to answer "Amen"]. You should know that "Amen, Yehei Shemei Rabboh [Mevorach Le'Olam Uleolmei Olmayoh]" has a higher priority than Kedushah. Therefore, if he came [into Shul] just before Kedushah and if he were to wait [to say his Amidah] until after Kedushah he would not be able to finish his Amidah until after Kaddish, but to wait [to say his Amidah] until after Kaddish he would be unable because the time for saying the Amidah would pass, then [in such circumstances] it is better to start his Amidah before Kedushah so that he will be able to answer "Amen, Yehei Shemei Rabboh.." [after the repetition]. Kedushah has a higher priority than "Modim". Therefore, if one came [into Shul] just before Kedushah and if one were to start [one's silent Amidah] immediately one would not be able to say Kedushah, but if one were to wait [to start one's Amidah] until after Kedushah one would then not be able to say "Modim" with the congregation, and to wait [to start one's Amidah] until after "Modim" is impossible because of a fear that the time for saying the Amidah would pass, [then in such circumstances] it is better that he starts his Amidah after Kedushah; so wrote the Magen Avrohom. But according to what we will explain below in MB 14 in the name of the Magen Giborim, it is better in this case for him to start [his Amidah] at the same time as the Leader [starts the repetition] and he will then have Kedushah and "Modim", and all the more so since "Amen, Yehei Shemei Rabboh.." has priority over "Modim". Therefore, one who comes into Shul immediately after Kedushah and if he were to wait [to start his Amidah] until after "Modim" he would not be able to answer "Amen, Yehei Shemei Rabboh..." in the Kaddish that comes after the Amidah, but if he were to wait until after Kaddish [to say his Amidah] the time for saying the Amidah would have passed, [then in such circumstances] he should start [his Amidah] immediately since it is better to dispense with "Modim" than with "Amen, Yehei Shemei Rabboh..", and furthermore it is possible to bow at "Modim" in the middle [of the Blessings [as mentioned in the Shulchan Aruch]. Similarly, "Amen, Yehei Shemei Rabboh.." has a higher priority than praying the Amidah with the congregation [Tefillah BeTzibur]. Therefore, if one came [into Shul] just before Kaddish [after the Minchah Amidah] and if he were to say the Amidah immediately then he would have to miss "Amen, Yehei Shemei Rabboh..", but if he were to wait until after Kaddish he would not be able to pray Ma'ariv with the congregation, [then in such circumstances] "Amen, Yehei Shemei Rabboh.." takes priority, and immediately after this he should start saying the Mincha Amidah, and he should not worry that the congregation has now started Ma'ariv - since it is still daytime. See what we have written further about this in the Biur Halachah in the name of the P'ri Megadim.

MB 5: And the same applies to Amen etc. - On the face of it [one could say] that that's fine as far as [waiting to say] "Amen" to "Shome'ah Tefillah" is concerned, in that there is a purpose [necessitating the teaching of this particular law] in that one needs to wait to say the Amidah, even after Kedushah, until one knows that one can complete [saying the Amidah] before the Blessing of "Shome'ah Tefillah". But as far as [waiting to say] "Amen" to "HoKel HaKodosh" is concerned, what practical difference or purpose is there [in teaching this particular law] since one is required to complete his Amidah before Kedushah, so it follows that one will be saying "Amen" after "HoKel Hakodosh" [at the end of Kedushah]? One could say that the practical difference is [in a case where] even though he has heard the Kedushah, nevertheless he is required to wait [to say his Amidah] because of [having to say the] "Amen" to "HoKel HaKodosh" which he has not yet heard. Also, it is a question of not beginning his Amidah immediately after Kedushah, but rather after he has answered "Amen" to the Blessing [of "HoKel HaKodosh"] because this is also connected to the Kedushah. The same law applies if he knows that the congregation will be saying "Borechu" and he hasn't heard it yet, if he is able to complete his Amidah before the congregation says "Borechu" then he may say the Amidah in the normal manner. But if he knows that he will not be able to complete his Amidah before [the congregation reach] "Borechu" then he is required to wait at "Shiroh Chadoshoh" until he has heard "Borechu". If he has already heard Kedushah or Borechu or he knows that he will be hearing [one or other] afterwards, then there is no need to wait [to say the Amidah]. For "Modim", however, even if he has heard it several times with the congregation he must still wait [to say his Amidah if he is unable to finish it before the Leader reaches "Modim"] because of the reason that he looks like one who denies [the existence of G-d] if he doesn't bow [with the congregation] (Later Rabbis). So also for Kaddish one must wait [to say one's Amidah] as there is no limit [to the number of Kaddishim one may say], although possibly the Kaddishim from "Oleinu" onwards are not included in this; so wrote the Magen Avrohom. But the Shaages Aryeh, the Derech HaChayim and the Chayei Odom wrote that with any of the Kaddishim, if one has already heard [one or other of them] and has answered "Amen, Yehei Shemei Rabboh.." then he no longer needs to wait [to say his Amidah in order to answer to any other Kaddish].

MB 6: After Kedushah - Not only [if he came in after Kedushah] but the same applies where he came into Shul before Kedushah and he waited to say the Amidah until after Kedushah, he is also required to estimate for himself whether he is able to complete [his Amidah] before the Leader reaches "Modim".

MB 7: Leader reaches "Modim" - According to the Rema mentioned above in this paragraph, he is required to complete [his Amidah] before [he will need to answer] "Amen" to "Shome'ah Tefillah", [which is only separated from "Modim" by the paragraph beginning "Retzei"] or [alternatively] he should estimate for himself as to whether he is able to reach "Shome'ah Tefillah" at one and the same time as the Leader [reaches it in the repetition]. But if not, he may not start his Amidah until [after] "Modim", and so the Later Rabbis decided is the Law.

MB 8: In which one bows etc. - Even though he is not able to say the "Modim D'Rabbonon" that the congregation says [at the same time as the Leader is saying "Modim"] that doesn't matter at all since he is bowing with them - and this is permitted, even in the first instance. But he should pace himself so that he says the Blessing of "Shome'ah Tefillah" at the same time as the Leader, as mentioned in paragraph 2.

MB 9: In order to have it come immediately after - The explanation of this case is that he erred [in not stopping at "Shiroh Chadoshoh" to wait to say his Amidah, but instead] said "Go'al Yisroel", as a result of which he is now required to say the Amidah immediately after [the Blessing of "Go'al Yisroel"]. The same applies if the time [for saying the Amidah] is passing and he is thus obligated to immediately say the Amidah, but otherwise he must in the first instance wait for the Kedushah at "Shiroh Chadoshoh", as is mentioned in Siman 66:9.

MB 10: Then he should bow with [the Leader] - But he does not say "Modim" then [as he may not interrupt his own silent Amidah]. The bowing is only in order that it should not look like he is denying the [existence of the] One to Whom the congregation are bowing. Nevertheless, if he is not forced to start [his Amidah] immediately then he is not permitted to start it with the intention of bowing in the middle of the Blessings, even though it is permitted to bow in the middle of all of the Blessings as will be mentioned later in Siman 113 - because perhaps he will forget to bow since he is not otherwise obligated to bow at that point, and if he tries to concentrate on [making sure he bows in the middle of a Blessing by keeping track of where in the repetition the Leader is] then he will not concentrate properly on his Amidah.