什么时候用现在分词什么时候用过去分词答:2、"while ( when, once, until, if , though等连词)+分词"结构现在分词或过去分词作状语时,有时可以在分词前加while,when, once, although, until, if等连词.When leaving the airport, she waved again and again to us. While waiting for the train, I had a long talk with my ...