1.因为前面有助动词does,所以后面跟原形have.比如 tom has short legs.（用has）x09tom doesn't have short legs.（有助动词doesn't,用have）x09does Tom have short legs?（有助动词does的问句,用have）
2.因为有助动词do并且they是复数,所以后面跟 原形.比如 they have apples.（因为they是复数,用have）x09do they have apples.（因为they是复数,并且因为助动词do的问句,肯定用have）