回答

(1) I <took off my coat> and <set to work>. ※< > and(等位接続詞) < >:and は<  >と<  >をつなぐ
(2) Night coming on, we started on our way home.
(3) Feeling very happy, Mary began to sing a song.
(4) Working very hard, he had no time to play.
(5) As(従位接続詞) I [had not been told](過去完了) how to drive his car, I was(過去) then at a loss.
※複文:従位接続詞+S+V~, S+V… /S+V~+従位接続詞+S+V…

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