Why a lefty wears tefillin on his right arm
A lefty who performs most of his work with his left arm/hand puts his tefillin on his right arm. Chazal derived this halacha in two ways. One is from the wording of "you shall tie them on your arm." The word used for arm in this pasuk is yadecha, written with a heh at the end to imply the weaker arm -- i.e. the tefillin should be tied on the weaker arm. The other is from the juxtaposition of the two words ukshartam and uchsavtam (you shall tie and you shall write). By putting these two words together, the Torah showed that just as writing is done with the right hand, the tefillin should be tied with the right hand. Someone who is capable of doing his work with his right arm/hand but finds it easier to perform the work with his left hand is considered a lefty.
(???? ?, ?"? ??, ?? ????)