I made mistakes in spelling ,=I made (spelling mistakes).
You shoud join an english club=You should (attend an english club).
What is difficult about learning English=What (troubles you) about learning English
He didn't konw how he could use the new mobile=He didn't konw (how to) use the new mobile
He became very good at sending fast messages=He was an (expert in) sending fast messafes
I found that it was difficult to learn English grammar=I found (it difficult) to learn English gramma