从语法角度来看，它并不严谨，就像前面朋友说的加个to或是把keep改成keeping，而且你这样翻译感觉相当的中式，不想native speaker会说的

可以给keep加ing，一个句子谓语动词只是一个

是不是应该在is和keep之间加个to啊

Colloquially, yes, OK, no need that 'to' after 'is' and before 'keep'. Strictly, that 'to' is required (or 'keeping') because you need an infinitive structure after verb 'be' (in this context), or adjective, noun or clause.

I think this erpression is ok