Sure! Here's the corrected version with the missing words filled in:
Went went (go) to school last week. I had (have) 3 exams. Math was very (be) very easy. Science and Civics were (be) very hard. I studied (study) a lot. My mom explained (explain) math to me. After school I ate (eat) pizza. I bought (buy) a new pair of tennis shoes. They were (be) nice.
Let me know if you'd like me to explain any part of this!