Can someone help me with Japanese Verbs?

Question:

So I've been learning Japanese for 3 years and I've started on Japanese verbs and i'm confused. I'm not sure how this whole "dictionary form" works. So, for instance to eat is "tabemasu" and then you would change it for instance to "tabemashita" to ate. but what does "taberu" for instance have anything to do with it. And when asking a question like "why are you eating meat?" why do you say
"Doshite niku o taberun desu ka?" instead of tabemasu

Sorry if i've explained my question badly or its really confusing but could someone please just give me a general overview on how verbs in Japanese work. It would be greatly appreciated. Thank you

Answer:

Taberu is a casual form of tabemasu. Past tense casual form is tabeta (casual of tabemashita).