1.第一句My favourite food is apples.这个句子是对的,陈述句的单复数,一定要看主语,这句话主语“food”是单数,所以,谓语要用“is”.
2、My favourite sport is football 就足以能表达你这句话的意思了,说我最喜欢的运动是足球,其实内涵就是踢足球的意思.要学会意会,英语和汉语不是一对一的.
如果用动词就应该用,现在分词playing football 现在分词短语用作表语,我觉得初一的水平知道My favourite sport is football 就足以了.
3,Hamburgers are my favourite food；Hamburgers are my favourite food.They are healthy.这两句是对的,主语都是hamburgers是复数所以谓语动词要用are!
最后希望你好好复习语法中主谓一致.