Why is Boruch Shem said quietly?

As Yaakov Avinu neared the end of his life, he gathered his children to reveal to them his knowledge of when the world would end.  At that moment he lost his ruach hakodesh and was unable to reveal the secret. Consequently, he became concerned that one of his children might have been a heretic. To convince him otherwise, they all said Shema Yisroel in unison. Upon hearing this affirmation of their faith, he responded with Boruch Shem, a praise recited by malachim. From this we see that it is appropriate to say Boruch Shem along with Shema. On the other hand, when Moshe Rabbeinu said Shema he omitted Boruch Shem, implying that it may be inappropriate for humans to say it. To solve this paradox, the custom has become to say Boruch Shem quietly. There are various other times in which Boruch Shem is incorporated in tefilah. The poskim disagree about whether it must be said quietly at those times. 

סעיף יג וס"ק ל; ביאורים ומוספים דרשו, 22

If one forgot Boruch Shem, must he go back to say it?

Boruch Shem is similar to the first pasuk of Shema in that it is also a part of kabalas ol malchus Shomayim. Therefore, if one did not have kavanah when saying Boruch Shem, he must say it again with kavanah. If he omitted it entirely, then it should be added at any point in Kriyas Shema when he remembers. One should pause slightly between the end of Boruch Shem and the beginning of V’ahavta to mark the distinction between kabalas ol malchus Shomayim and the rest of the mitzvos. A pause should also be made between the first pasuk and Boruch Shem, to show a difference between the primary acceptance and the secondary kabbalah.

סעיף יד, ס"ק לא, וביה"ל ד"ה אחר; ביאורים ומוספים דרשו, 20 ,וראה שם, 2

Can one respond Amen to a brocha twice?

One should not say the word Amen twice when responding to a brocha. It gives the impression that there could be two powers (chas v’sholom). If one hears two brochos completed at the same time, one Amen suffices for both. One may also answer Amen v’Amen. In a case where the brocha contains both a praise and a request and the Amen has two separate meanings (to affirm and to request fulfillment), repeating the Amen is permitted. An example of such a brocha would be Refoeinu, which praises Hashem as the true healer and requests that He heal the sick.

סעיף יב וס"ק כח; ביאורים ומוספים דרשו, 19

• When saying the word echad in Shema, one should bear in mind the following: that Hashem is one, that He is the King of both heaven and earth, and that His rule extends across all of the four directions.

• In some places it is customary for a person to move his head in the four cardinal directions when saying the word echad, to show that Hashem’s rulership extends to all places.

• It is forbidden to repeat the words of the pasuk Shema Yisroel. This includes both repeating individual words and the entire pasuk. 

• Can one be yotzei by listening to another person say Kriyas Shema?

• The importance of reading Loshon Kodesh correctly

• Why do some people kiss their hand after touching their tefillin?