Употребить Present, Past или Future Perfect.

1. Не (to study) French before he (to enter) the university.

2. When we (to come) to the station, the train already (to leave).

3. I (to do) my homework by six o'clock tomorrow.

He had studied French before he entered the university.When we came to the station, the train had already left.I will have done my homework six o'clock tomorrow.