funny how that differs depending on your native language; I can hear the 'un' perfectly
When I had first started studying Italian, I too wondered how people are supposed to pick up on those little words like "un", "di", "l' " and so forth. All I can say is with enough exposure you just develop the ear for it. Certain things just won't sound right anymore without other things. Time, patience, and a lot of listening will solve that problem.
The alternative is often people talking with food in their mouths. Or dealing with the awkward pausing while they chew and swallow. I'll pass.
I don't think it have a feminine form in singular nor plural...pasta would mean pasta and paste, according to Google translate, does not seems to exist, as it just keep asking me whether I'm translating from English.