1. He helped me a lot.
= He often/always/usually helped me.
= He offered/gave me a lot of help.
2. They have been friends for 6 years.（改为同义句）
= They have been friend （since 6 years ago）.
3. I have kept this book since 4 days ago.(同上）
= I (borrowed) this book (4 days ago).
4. He helped me a lot同义句
见第一题.
(With his) help, I could pass the exam.