1.谓语是be的情况下用were、was:
I am a student in XX university 我是XX大学的学生（现在时态）；I was a student in XX university 我曾经是XX大学的学生（过去时态）
2.谓语是其他动词的否定句情况下用did：
I study in XX university; I did not study in XX university.
3.谓语是其他动词的强调句情况下用did：
I do understand it 我确实明白了；I did understand it 我过去确实明白了.