一般过去时和现在完成时怎样区分？

一般过去时，一般都有过去的的时间：last year；last week……例如：he did his homework last week。 现在完成时，一般都是强调已经完成了。而且有明显的词：already，been，gone，for……例如：I've been doing homework since 8 o'clock. 求采纳，手打的