eat,watch是原形,用在助动词后,如：
Does he eat meat?
He doesn't watch TV on work days.
eats,watches是三单,用于第三人称单数,如：
He usually eats meat for lunch.
He watches TV only at weekends.
eating,watching是现在分词,一般用于进行时,如：
He's eating dinner.
He's watching TV.