Замените придаточное предложение причины причастным оборотами:
1) As he thought that it was his brother at the window. Steve deceided to open it.
2) Since he knew who the man was, make was very pleased to have the chance of talking to him.

1) Thinking that it was his brother at the window, Steve decided to open it.2) Knowing who the man was, Robert was very pleased to have the chance of talking to him