能说"Write in English if possible"吗?

Sure ..yopu can have another way to express out ..for eg.
"In English ..please" ...so either you speak or write ..both are ok.
如果可以的话请"用"英语.if不是只能连接句子吗？（老师强调过）i have no idea your 连接句子 .. let me show Usome examples on IF 'IF'' is noun and conjunction. A conjunction is a joining word. - for eg . IF, Whether, Although etc1.If she goes out, she will lock the door. In situation hasn't happened yet, and we can easily imagine it happening2. If I am late, I will call you. First part is going to happen. Second part says what will happen as a result.3. My brother will help you if he can. - - is this what u meant ? - 连接句子 ?4. Do you mind if i help you ?IF CAN BE AT BEGINNING AND MIDDLE OF THE SENTENCES ! ok now ?