Should one interrupt in the middle of Pesukei D’zimrah to say Shema?

A person who walked into a shul while the tzibur was saying Kriyas Shema must join them in the recitation of the pasuk of Shema together with Boruch Shem. Otherwise, it will appear as if he is uninterested in being mekabel ol malchus Shomayim together with the tzibur. According to some Achronim, he must recite the entire Kriyas Shema with them. If he has not yet davened, he should have in mind not to be yotzei so that he can fulfill the mitzvah properly by saying it with the brochos. If one has not yet said Birchos HaTorah, he should only say Shema and Boruch Shem. If one was in the middle of Pesukei D’zimrah when the tzibur reached Shema, Shulchon Aruch rules that he should not interrupt. Instead, he should read the words in the same tune which the people are using to say Shema. Most other Achronim hold that one should say Shema even in the middle of Pesukei D’zimrah.

סעיף ב-ג וס"ק ח, י ו־יא; ביאורים ומוספים דרשו, 15 ו־20

Must one respond to Shema during Kedusha of Musaf?

There are other times in which the tzibur says Shema for reasons other than to be yotzei the obligation of Kriyas Shema (e.g. when the Sefer Torah is taken out for Kriyas HaTorah, during Kedusha of Musaf and during Yom Kippur Koton). The poskim disagree about whether one must say it together with the tzibur if he enters the shul at that point. Some say that he should, unless he is in the middle of Kriyas Shema. Some say that if the tzibur is saying Shema after the proper time, he does not need to say it together with them.

ביאורים ומוספים דרשו, 18 ו־22

Must Aleinu be said with the tzibur?

It is appropriate to join with the tzibur when they are saying certain tefillos, including Aleinu and Ashrei. If one is davening with the tzibur but is still saying the tefillos prior to Aleinu, however, he does not need to stop in order to join them. Nusach Ashkenaz recites Aleinu before saying the Yom, while Nusach Sefard says it as the last part of davening. One who is davening in a congregation whose nusach is different than his own should say Aleinu together with the tzibur. Some poskim say that one who davens Nusach Sefard should be sure to recite Aleinu as the last tefilah even if he already said it previously with the tzibur.

ס"ק ט ביאורים ומוספים דרשו, 17

• While saying certain tefillos or brochos, a person may not take a break and remain quiet for the amount of time that it takes to recite that particular passage from beginning to end. These tefillos include any brocha, birchos Kriyas Shema, Shema, Shemoneh Esrei, Hallel or Megilas Esther.

• B’dieved, if one took a break in the middle of saying Shema for the amount of time it takes to say all three parshios, he is yotzei according the Shulchon Aruch and he does not have to say Shema again. Rema holds that if he was forced to take the extended break for halachic reasons (e.g. he needed to use the restroom or the area became unclean), he must say Shema again.

• If a person took an extended break in the middle of Shemoneh Esrei, the Shulchon Aruch holds that he must repeat it even if the interruption was not halachically mandated. Rema maintains that Shemoneh Esrei only has to be repeated if one was forced to interrupt it. 

• Who may one greet in the middle of saying Shema?

• Do people get insulted if they are ignored during davening?

• May one write when it is forbidden to speak?