英语翻译用can may must翻译：他当时不可能生病.他当时不可能看过这部电影.他不可能一直在看书.

He couldn‘t be ill then.(强调那一刻的状态)如果不强调,一般的就是He wasn't ill then.
He couldn't have seen this movie then.过去完成时,强调在过去的一点他没做过,如果不强调,一般的就是He hadn't seen this movie then.
He couldn't have been reeading all the time.如果不强调,一般的就是He hasn't been reading all the time.
三单can变could,因为情态动词只接原型,即can/could+do,
如果直接变有疑惑的话,先写成一般式,再对照着句子成分把do/did之类的换成can/could