Who may one greet in the middle of saying Shema?

Each of the parshiyos and brochos of Kriyas Shema are known as prakim, and the points between them are known as bein haprakim. If one is bein haprakim, he may greet a respectable person (e.g. an elderly person or talmid chochom). One may also return anyone’s greeting. In the middle of a perek (except for the pesukim of Shema and Boruch Shem), one may only greet a person whom he reveres, such as a parent or primary rebbi. He may also return the greeting of a respectable person. It is permissible to greet a powerful person in the middle of a perek, too, if ignoring him could result in a material loss. These halachos are recorded in the Gemara and Shulchon Aruch. The practical halacha, however, is discussed below.

סעיף א, ס"ק ג, ד, ה, ט ו־יא, וביה"ל ד"ה או, וראה שם ד"ה שלא

Do people get insulted if they are ignored during davening?

The above halachos of greeting people during davening applied only in an age when it was more common to greet people (even during davening). At that time, neglecting to greet someone was insulting. The poskim say that since greeting people during davening is not socially accepted today and people no longer expect to be acknowledged that way, it is forbidden to greet people or return their greetings while one is davening. The sole case in which answering a greeting is still pertinent is when a gabbai asks for the name of a person who is davening in order to arrange for an aliyah. It would be disrespectful to ignore him.

ס"ק ב; ביאורים ומוספים דרשו, 4

May one write when it is forbidden to speak?

The poskim extensively discuss whether writing is the equivalent of speaking. One case which is subject to debate is whether a person can be yotzei sefiras haomer by writing down the count of the day. The poskim extend this debate to writing at points during davening when it is forbidden to speak. Some say that writing is more strictly forbidden than speaking. Others say that writing is only forbidden because when a person writes, he is susceptible to blurting out a word. A third opinion permits writing at points when it is forbidden to speak.

ביאורים ומוספים דרשו, 2

• 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.

• 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.

• 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. 

• Which brochos should one respond to during Shema?

• Can one say the entire Modim D’rabonon when he is in the middle of Shema?

• Should a brocha be said over a tallis during Shema?