1. If he (not to be) ill yesterday, he (to come) to school.

Решение:

Это условное предложение 2-го типа, которое описывает нереальную ситуацию в прошлом. Используем Past Simple в первой части и 'would/could/might + инфинитив' во второй.

1. If he had not been ill yesterday, he would have come to school.

Ответ: If he had not been ill yesterday, he would have come to school.