too和also一般都用于肯定句,too放在句尾,通常用句号隔开,also放在实义动词前,联系动词后.例：He runs fast,too.=He also runs fast. I'm a student,too.=I'm also a student. either主要用于否定句,放在句尾,通常用逗号隔开.例：I don't know it,either. 注意：另外too还有“太..以至于不能.. ”的用法.例：He's too old to go. The lesson is too difficult to understand.