have當一般動詞時,後面為了不重複就會用助動詞代替,my brother是三單,故用does代替has(不是have)。
have當助動詞時(完成式),後面才會用have:I have eaten breakfast, but my brother hasn't.
一般動詞就用do取代,三單用does,過去式用did:I don't teach English, but my brother does.(my brother三單用does取代)
I didn't teach English, but my brother did.(過去式)
be動詞後面用be動詞(過去式也一樣):I am a teacher, but my brother isn't.
I was a teacher, but my brother wasn't.