sometimes,he and she watch television这句是对的.
he and she are watch television是不对的,应改为he and she are watching television.正在看电视.
am is are是be动词,在句子中充当动词成分,watch也是动词,所以不能出现在一个句子里.be+doing是现在进行时的形式,所以是are watching television.
特别是一般现在式你要注意,有am is are 就不可以有其它动词出现在一个句子里.