for和since是现在完成时的提示词汇,但两者用法不同:(1)for和表示一段时间的词组连用,如: for three minutes, for six hours, for nine days for twelve weeks, for fifteen months, for thirty years(2)since和表示过去某一时刻的词或词组连用,如: since nine o'clock this morning, since three weeks ago since last Friday, since September