主要通过谓语动词、时间状语来判断,举几个例子来说吧:
** 主语+V(单三)+宾语 一般现在时
she goes to school on foot.
i love feeding animals.
(一般现在是表示经常发生的事)
这两句中的谓语动词go(goes)和love,都是动词原形或单三形式,可判断是一般现在时;
** 主语+Ved(was、were. etc)一般过去时
Rose lost her purse on the way to workplace yesterday.
he was very angry and left home.
两句中的lost、left、was分别是lose、leave、be的过去式,当然还有yesterday明显的时间状语,可判断是一般过去时;
** be+doing 现在进行时
she is singing a song now.
** was/were+doing 过去进行时
he was watching TV when we visited him.我们拜访他的时候他正在看电视.
我们拜访的时候本身就发生在过去,他当时正在做的自然是过去进行时了;
** will+do/ is(are) going to do 一般将来时
i am going to attend a speeh this evening.我今晚要去参加一个演讲.
** would/ was(were) going to do 过去将来时
he said he would go for a walk the next day.他说他明天要去散步.