1. The verb root of seen is see.

2. The root verb of seen is see.
------------------------------
Do we have to use 'the verb root' or 'the root verb'? Which one is grammatical? I can see 'verb root' a lot on the Internet. However, I can hardly see 'root verb'. On youtube, I can find 'root verb' from lecturers.

#2 is correct. I've never heard the other term used.