או"ח א' סימן ב'
הכותב בשמאל ורוב מלאכתו בימין על איזו יד יניח תפילין
Generally, according to the Rema's ruling in Shulchan Aruch Orach Chaim (Siman 27 Seif 6), one who writes with their left hand and does most of their actions with their right should put on tefillin on their right hand. This follows the opinion that the hand used for writing determines the placement. However, there is a differing opinion, notably from the Gra, which considers the hand used for the majority of actions. Therefore, it is considered proper for one who fears God to also place tefillin on the left hand after the main prayer, following the Gra's view. For a young boy just starting to put on tefillin, placing them on the right hand is sufficient. The Shulchan Aruch discusses various opinions: some say to place tefillin on the weaker hand, while others, like the Tur and Smag, prioritize the writing hand, implying that the hand used for writing dictates that tefillin be placed on the opposite hand. The Grish Eliashiv also ruled in accordance with the Rema's view. When switching tefillin after prayer, only the hand tefillin needs to be changed, ensuring the 'Yud' faces the heart.
Generally, according to the Rema's ruling in Shulchan Aruch Orach Chaim (Siman 27 Seif 6), one who writes with their left hand and does most of their actions with their right should put on tefillin on their right hand. This follows the opinion that the hand used for writing determines the placement. However, there is a differing opinion, notably from the Gra, which considers the hand used for the majority of actions. Therefore, it is considered proper for one who fears God to also place tefillin on the left hand after the main prayer, following the Gra's view. For a young boy just starting to put on tefillin, placing them on the right hand is sufficient. The Shulchan Aruch discusses various opinions: some say to place tefillin on the weaker hand, while others, like the Tur and Smag, prioritize the writing hand, implying that the hand used for writing dictates that tefillin be placed on the opposite hand. The Grish Eliashiv also ruled in accordance with the Rema's view. When switching tefillin after prayer, only the hand tefillin needs to be changed, ensuring the 'Yud' faces the heart.