definite, article,the,a, an

[huda]

hi,
im facing a problem with the, a, an.
by the fact, regarding ; an ,i can know when I could use it.
but regarding a, im not sure.
of course, i know it is using with sound countable singular but not always.
sometimes , it doesn't used

is so, when?
as well as for the , Im not sure when I have to use it.
i need strong rule for that.
thanks