We need to make some corrections. The first word of a sentence is always capitalized. The comma isn't correct, either it needs to be a semicolon, or we need to break the sentence into two sentences. We need to fix some other problems. 1) He had a fever yesterday; he couldn't go to school. 2) He had a fever yesterday; he couldn't have gone to school. Both of these sentences are correct. Both of these sentences are in the past. Sentence 1 has a simple meaning. It explains why he wasn't at school today. Sentence 2 is complicated. It would be used in a context like this: "I thought I saw him at school yesterday." "He had a fever yesterday; he couldn't have gone to school." The second sentence means "you must be wrong. What you thought you saw was an impossibility."