情态动词的过去式主要是关于COULD的You------ him at the party last night.(CA

应该这样说：表示猜测时,can以及can的过去式could都可以对过去或者现在的动作、状态进行否定的猜测.但后者语气弱一些.
对现在进行猜测：can/could not be doing sth.
对过去进行测测：can/could not have done sth.
根据猜测的把握性大小进行排列：
must have done sth.
can't have done sth.
could have done sth.
may have done sth
might have done sth.
这种用法中,情态动词本身的时态形式和动作发生的时间没有关系.