USE OF a and the

[CONNECTBRAIN]

I know "a" is used before a singular countable noun and "the" when we say something or refer to something in specific and for singular and plural countable noun.But I get confused using it can you please elaborate on the use of "a" and "the"
Please correct these sentences grammatically( Is the use of a correct over here and why)
1-I'm going to take a shower
2-I was in the shower when you called.
3-I'm going for a salsa night
4-On the way to the exams my bike got punctured.

 

[2010]

I know "a" is used before a singular countable noun and "the" when we say something or refer to something in specific and for singular and plural countable noun.But I get confused using it can you please elaborate on the use of "a" and "the"
Please correct these sentences grammatically( Is the use of a correct over here and why)
1-I'm going to take a shower (I took shower twice today)
2-I was in the shower when you called. (you know where you were at that time)
3-I'm going for a salsa night (You are not going for more than one Salsa)
4-On the way to the exams my bike tyre got punctured.

===Not a teacher===

Further explanation to this by a teacher would be very helpful.