is是be动词.
be动词包括：am , is , are.
如果把一个句子变成疑问句,首先要看这个句子的动词是什么.
如果句中有be动词,直接把be动词提前.
例如：
This is a pen.有be动词,直接提前,就是Is this a pen?
They are eating apples. Are they eating apples?
如果句中没有be动词,而是实意动词,就得借助于助动词do 或 does 提问.
例如：
They had an oral english class yesterday.
had 是实意动词.借助助动词提问： Did they have an oral english class yesterday?
had 是过去式,所以用did 提问.变为疑问句的时候动词had 要还原为have.
He goes to school by bus everyday. go 是实意动词,就得借助助动词does提问.变为：Does he go to school by bus everyday?
he 是第三人称单数,所以用does 提问,疑问句中goes还原为go