Can a daled be extended?

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. Some poskim say that one should extend the pronunciation of both the ches and the daled in order to incorporate these thoughts. Others say not to extend the ches but to contemplate these ideas when extending the daled. The poskim say that if it is difficult for a person to have all this in mind while saying echad, he can reflect on the ideas immediately after saying the word. In addition, they say that given the common contemporary pronunciation of a daled, it is difficult to stretch it to the extent that all these kavonos can be incorporated.

סעיף ו וס"ק יח; ביאורים ומוספים דרשו, 11

What is an improper way to shake one’s head?

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. The proper order is to go from east to north and then west to south. One should not go from east to west and then north to south, as it would appear as if he is making a cross.

סעיף ו וס"ק כ, עם הרחבת המקור שבשעה"צ ס"ק י

Is repeating Shema ever permitted?

It is forbidden to repeat the words of the pasuk Shema Yisroel. This includes both repeating individual words and the entire pasuk. Saying the words (or pasuk) twice gives the impression that one is acknowledging (G-d forbid) two powers. If one did not have kavanah when saying Shema the first time, he should repeat it quietly if there are other people in the vicinity. If he is by himself, he may say it out loud. It is also forbidden to repeat any pasuk from the entire first parsha. When saying Kriyas Shema al Hamitah, however, one may repeat the first parsha if repetition helps one fall asleep. The first pasuk may never be repeated unless one is reciting all three parshios of the nighttime Shema.  The poskim disagree about whether repeating the pasuk of Shema is permitted when it is reviewed during shnayim Mikrah. Some say that one should say it quietly the second time.

סעיף ט-י, וס"ק כב, כד ו־כה; ביאורים ומוספים דרשו, 16-15

• The Midrash states that saying the 248 words of Kriyas Shema can heal a person's 248 body parts. There are, however, only 245 words in Shema. As a solution to this problem, the chazan repeats the words Hashem Elokeichem emes. Since he is motzei the tzibur, it is as if everyone said all 248 words.

• A person who is davening by himself does not have a chazan to complete the necessary word count. Shulchon Aruch says that a person in this situation should keep certain numerical calculations in mind when saying the brocha of V’yatziv. Rema offers another option of saying the words Kel Melech Ne’eman just before Shema.

• Some Achronim say that if a person is still in the middle of Shema when the chazan is done, he should stop and listen to the chazan say Hashem Elokeichem emes.

• Why is Boruch Shem said quietly?

• If one forgot Boruch Shem, must he go back to say it in its proper place?

• Can one respond Amen to a brocha twice?