Could you help me when I make mistakes in English?
I would really like you to help me when I stumble in English
Could you help me when I trip up in English?
「助けて欲しい」は「want you to help me」という文法になりますが、これは少しダイレクト過ぎてあまり丁寧な頼み方ではない。ですから、「Could you help me?」や「I would really like you to help me」という丁寧な頼み方にした方が良いと思います。^^
「つまずく」= stumble/trip up/fail/make a mistake
「もし僕がつまづいた時は助けて欲しい」= Could you help me when I make mistakes in English? / I would really like you to help me when I stumble in English / Could you help me when I trip up in English?