?1. 表示經(jīng)常性，習(xí)慣性,永久性的動(dòng)作或存在的狀態(tài).
通常與副詞sometimes, often, usually, always, every day (year, month ), once (twice, three times) a day,等時(shí)間狀語連用。
They usually go to school by bike.
I take the medicine three times a day.
She helps her mother once a week.
Mary’s father is a policeman.
There are 50 students in my class.
2. 表示客觀真理，科學(xué)原理，自然現(xiàn)象，等客觀事實(shí)或格言，諺語等。
The sun rises in the east and sets in the west every day.
The man who has never been to the Great Wall is not a real man. Tomorrow is Tuesday.
3 表將來，(將來發(fā)生的這件事是在日程表中一定會(huì)發(fā)生時(shí))
The plane takes off at 4:30.
(當(dāng)用于if, when等狀語從句，主句將來時(shí)，從句現(xiàn)在時(shí)，現(xiàn)在表將來。)
If you study hard, you will succeed.