你好.简要地说：
1 if only == how I wish - - -- ! --------- 用于虚拟语气的句子：
If only she knew I need her! (but she doesn't know I need her.)
If only she could have lived a little longer! (but she didn't stay long.)
2 only if -------- 用于倒装句,条件状语从句：
Only if I have enough time, will I go with you. ===If I have enough time. I will go with you.
Only if we all help, shall all be saved. ==== If we all help, all (the animals) will be saved.