当since 解释为“自从”时,用于现在完成时,通常后跟时间,ago 以及一般过去式的句子.例句；China has taken place great changes since 1978.They have gone to Hong Kong since two days ago.It is(has been) twenty years since they moved here.
since 还有另一层意思“既然,由于”,时态视具体情况而定.
当since 解释为“自从”时,用于现在完成时,通常后跟时间,ago 以及一般过去式的句子.例句；China has taken place great changes since 1978.They have gone to Hong Kong since two days ago.It is(has been) twenty years since they moved here.
since 还有另一层意思“既然,由于”,时态视具体情况而定.