英语翻译：不知道怎么表达的时候就用英语来表达！

Speak English when you don't know how to express yourself.

If you have difficulty expressing something, use English.
教老外学汉语呢吧。

You can express yourself in English whenever you don not know how to express

You can use English to express yourself if you don't know how to express.

if you don't know how to express something you can use english