Welcome!
I don't speak much German, but I do speak French and your teacher is right -- in order to be fluent in a language, you have to be able to think in it. It's a bit strange, really, but it becomes natural quite quickly.
If you have any questions about English grammar, go
here; we have a lot of people who are very knowledgeable on the subject in our community, and we'll be happy to help you!