两者都属现在一般时.

　　isn't系标结构,用于（第三人称）单数,比如：S/heisn'tastudent/Itisn'thisfault.

　　don't动词否定式,用于第一人称单数,及第二,三人称复数,比如：Idon'tknow/Theydon'thaveanyideasaboutwhathashappenedattheparty.