Question

Whether "the" or "a" is used before the word university ...but in some instance 'the' comes why???

Answer 1

'The' is used when you already know it.

'A' is used to introduce it.

Answer 2

'A' is used in university because if you were to split the word according to pronunciations, then it would be " YOU-NI-VERSE-ITY". Here the 'U' gives a pronunciation of YOU, that is Y. So, university has an "A" put before it. Again, "A" is used only when it is not specifying to something.

EXAMPLE:  she is studying in a university

                   here, they haven't mentioned which university, so "A" is used. But if the sentence were to be she is studying in the presidency university, then "the" is used because the have mentioned which university she is studying in.