Basically every verb in Gaelic has a positive form, a negative form, and a question. Generally those three are basically the same. But you get something called irregular verbs (which is where the words differ for various reasons).
What you have here with "a bheil" is the question form of tha. And, the reason it looks nothing like the word tha is because tha is an irregular verb.
Tha - positive chan eil - negative a bheil? - question
with a regular verb (which is a solid 90% plus of the language's vocabulary probably) it would look like this.
Leugh - positive Cha do leugh - negative an do leugh - question
(leugh means read by the way)
Though it should be mentioned that any other verb than 'is' will either be the past tense, or the future tense (which is also used for the habitual present) because 'is' is the only verb with a present tense. Hence all the at doing, at going etc. So leugh mi an leabhar - I read the book (past tense), Cha do leugh an leabhar - I didn't read the book, an do leugh an leabhar - did you read the book? (and just for fun, nach do leugh an leabhar - didn't you read the book?). But we'll be likely getting to that in time.