I grabbed my boyfriend by the arm to stop him from getting into a fight with another guy.
ーI grabbed my boyfriend by the arm to stop him from getting into a fight with another guy.
「私は彼氏の腕をつかみ、相手の男性と喧嘩をしないよう止めた」
to grab someone by the arm で「人の腕をつかむ」
to stop someone from ... で「人が…するのをやめさせる・妨げる」
to get into a fight で「ケンカになる・ケンカをする」
ご参考まで!