why sometimes the 'issa, iksan etc.' are omitted?
Could you give an example of a sentence where it was ommitted?
because each one of them have different functions in a sentence. find out what they are used for.
because verbs don't need it
I can't write that y
You might want to go with "yy" then. (Works for me.)
Taoba liris instead of taoba liris issa