Tfilin Shel Yad
1. The Tfilin of the arm should be placed properly on the left arm. This is on the place of the biceps, the muscle of the upper arm. The Mishna Brura prefers placing them on the high part of the muscle. If one has only large Tfilin he should place them higher toward the armpit rather than lower down, for the area around the elbow is not valid for Tfilin.
2. Regarding one who, G-d forbid, has no left arm (it has been amputated, etc.) the following applies:
a. If the upper arm and even part of the forearm still remain he may even recite a brocha.
b. If none of the forearm remains, a brocha is not said but perhaps two brochos may be recited on the “shel Rosh”.
c. If only a stump remains above the biceps area or nothing remains, he is exempt from wearing Tfilin even on the right arm. Some say he should place them on the right arm.
d. If one’s right arm was severed but the area of the biceps remains and now the person performs all activities with the left arm, his right arm is now the place for Tfilin.
3. It is most important to place the knot-like yud of the Tfilin towards the heart. We also place the pass through of the straps towards the shoulder and the Tfilin boxes toward the hand. A right-handed person who only has “lefty” Tfilin, or vice versa, should wear the Tfilin with the “yud” towards the heart, although, then, the Tfilin would be facing upwards with the pass through downwards.
4. If one must place Tfilin upon a bandaged upper arm he should recite two brochos on the “shel yad”.
5. Concerning a person who is left handed:
a. He places Tfilin on the right hand.
b. If he is ambidextrous he places them on the left hand
c. If he writes with the left hand but does other activities with the right hand, and if he also could easily do activity with the left hand then he would place them on the left hand.
d. However, if he only writes with the left hand and the other activities only with the right hand the Halacha is a controversy among many poskim.