Is it necessary to recite an entire brocha?

When saying a brocha, one may not diverge from the text that Chazal composed. In the literature of Chazal, however, we find that they wrote only the beginnings and ends of brochos and determined whether the brochos would be long or short. Regarding the middle of brochos, however, we do not find exact texts from Chazal. Therefore, if one omitted or changed the nusach that is found in the siddur for the middle of the brocha, he is yotzei the brocha bedieved if he mentioned the primary concept of the brocha. There are some Rishonim who say that if one omitted the end of a long brocha, he is still yotzei. L’chatchila, though, one must say the entire brocha as it is printed in the siddur and as it was transmitted through the generations.

ס"ק א; ביאורים ומוספים דרשו 1-2,   

Can a person change his nusach?

As the generations progressed, various nuschos hatefilah developed. According to the teachings of Kabbalah, there are twelve gates of tefilah in Shomayim, corresponding to the twelve shevatim. The Ashkenazic and Sephardic communities each have their own nusach. The Sephardic one is known as Edot Hamizrach. Ashkenazim and Sephardim also have different pronunciations of Lashon Kodesh. The poskim are critical of Ashkenazim who adopt the pronunciation of Sephardim. In the communities that follow the teachings of the Arizal (primarily Chassidim), a nusach known as Nusach Sefard developed. This nusach incorporates certain kavonos into the text of Nusach Ashkenaz. The poskim disagree about whether one may change from Nusach Ashkenaz to Nusach Sefard or vice versa. Some say that one may change from Nusach Sefard to Ashkenaz but not the reverse. They explain that changing from Sefard to Ashkenaz is not a change from the custom of one’s forefathers. Since those who daven Nusach Sefard are generally Ashkenazim, the ancestor who changed to Sefard diverged from this heritage. The descendant (who is adopting Nusach Ashkenaz) is merely reverting back to the family nusach. Others argue that it is acceptable to change to Sefard. Some even hold that a person may only change to Sefard. They explain that since the Arizal said that his nusach can enter through any gate in Heaven, it is the superior nusach.

ס"ק ד' ביאורים ומוספים דרשו 6

May one follow a nusach other than the tzibur’s?

There are various opinions among the poskim as to how a person should conduct himself when he davens in a shul which does not follow his nusach:  1) For the parts of davening which are said aloud, one must follow the shul’s nusach. During the quiet parts, one may daven in his own nusach. 2) One may daven the entire davening in his own nusach, except for the parts which are only said with a tzibur (e.g. Kedusha). 3) The entire davening must follow the shul’s nusach, except for Shemoneh Esrei. A chazan must always follow the nusach of the tzibur.

ביאורים ומוספים דרשו, 8

• If a person is not sure whether he is required to do a mitzvah min haTorah, the halacha is that he must do the mitzvah. Uncertainty in this case includes the question of whether one is obligated in the mitzvah as well as doubts about whether one had already done the mitzvah. Another example of uncertainty involves situations where a Torah mitzvah was supposed to have been done during the day, but it is already bein hashmoshos.

• If a person is not sure whether he recited Kriyas Shema and its brochos, Shulchon Aruch rules that he must recite both Shema and the brochos.

• There is a Torah obligation to mention yetzias Mitzrayim once during the day and once at night. Although this mitzvah can be done any time throughout the day by saying any pasuk or halacha related to yetzias Mitzrayim, Chazal instituted that it should be performed before the end of zman Kriyas Shema when saying the third parsha of Shema. 

• To qualify as tefilah b’tzibur, must all ten participants in a minyan be davening?

• How can a person who missed a minyan still hear Kaddish and Borchu?

• Why should Borchu be said at the end of davening?