is是be動詞.
be動詞包括：am , is , are.
如果把一個句子變成疑問句,首先要看這個句子的動詞是什么.
如果句中有be動詞,直接把be動詞提前.
例如：
This is a pen.有be動詞,直接提前,就是Is this a pen?
They are eating apples. Are they eating apples?
如果句中沒有be動詞,而是實意動詞,就得借助于助動詞do 或 does 提問.
例如：
They had an oral english class yesterday.
had 是實意動詞.借助助動詞提問： Did they have an oral english class yesterday?
had 是過去式,所以用did 提問.變?yōu)橐蓡柧涞臅r候動詞had 要還原為have.
He goes to school by bus everyday. go 是實意動詞,就得借助助動詞does提問.變?yōu)椋篋oes he go to school by bus everyday?
he 是第三人稱單數(shù),所以用does 提問,疑問句中g(shù)oes還原為go