1. 表示经常性，习惯性,永久性的动作或存在的状态.
通常与副词sometimes, often, usually, always, every day (year, month ), once (twice, three times) a day,等时间状语连用。
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. 表示客观真理，科学原理，自然现象，等客观事实或格言，谚语等。
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 表将来，(将来发生的这件事是在日程表中一定会发生时)
The plane takes off at 4:30.
(当用于if, when等状语从句，主句将来时，从句现在时，现在表将来。)
If you study hard, you will succeed.