yingyu often yong fa

watches 第三人称

应该是
he often watches TV

often
adv.
多次； 时常
How often do you go there?
你多久去那里一次？
How often you have been abroad? No often, only twice.
你出国几次了？不多，只出去过两次。

经常；通常
Children often dislike homework.
孩子们通常都不喜欢家庭作业。
He often does this.'
它经常干这种事。'

he often watches TV

Watches often只表示频率的副词，不是情态动词。所以单数第三人称形式不变！