I don't want this big fish.
Do you want this big fish?
What do you want?
She wants this big fish.
句意:
我想要这条大鱼。
含有实义动词want 的肯定句改为否定句,主语为I,故用助动词don't,后跟动词原形want,再接其他,即:I don't want this big fish.
含有实义动词want 的肯定句改为一般疑问句,原句主语为I,助动词用do放句首,I变you,动词用原形want,再接其他,即:Do you want this big fish?
画线部分“this big fish”意思是这条大鱼,可知问的是事物,用What提问,后跟助动词do,I变you,再接动词原形want,即:What do you want?
主语改为she:主语she为第三人称单数,谓语动词保持一致用三单形式wants,其他不变,即:She wants this big fish.