Question

why do we use 'For *a* long time....'and not 'For *the* long time....'.?why A and not THE?

Answer 1

we use 'a' for things that are not been talked as particular and 'the' for particular things.
so long time is npt particulsr it can be anywhere at anytime so we use 'a' fpr long time as it is npt particular.

Hope it helps