副词可以充当时间状语。
例如：
immediately， directly， instantly，early，late，before，later，yet
already，just，sometimes，often，usually，once，twice、three times...
I came immediately you called.
Directly I walked in the door I smelt smoke.
We will visit the Great Wall tomorrow.
Sometimes I get up early.
The workers usually have lunch at the factory.
Take this medicine twice a day.
