The -un ending on 'samak' gives the meaning that it's indefinite ('a fish' rather than 'the fish'), and that it's in the nominative case (so if it were in a complete sentence, the fish would be doing something or being described, rather than, say, being eaten).

Standard Arabic has cases. This is the nominative case. And no, it doesn't actually indicate indefiniteness, but it can't come together with the definite article. So in محمدٌ رسول الله ("Muhammad is the messenger of Allah," part of the Muslim creed of faith), Muhammad is also followed by -un, even though it is definite. But if we were to say الباردُ (the cold one), we'd simply attach an -u sound to the end, without the -n.

In practice, many native speakers don't use the case endings, or use them inconsistently or incorrectly, because the modern vernaculars don't use those case endings. So if it makes it easier for you, you can simply ignore the ending, and you'll be understood just fine.

this is solely a casing and grammar issue that reflects on pronunciation, tanween or 'nunation' and can take three forms: un, an or in

probably will be covered at a later stage. Best not to think of it as reflective of meaning at this point.

We generally omit the ending in pronunciation at the end of the utterance. So you're actually right, it should be samakun baaridun, but we'd pronounce it samakun baarid, unless it was followed by something else, like, say, samakun baaridun ladhiidh (delicious, cold fish).

Do remember that Arabic has two "aa" sounds, one like the one in American English "mat" and one like the one in American English "father." None like the French or Spanish A sound. So it can sound like "berid" to somebody whose native language has a more "central" A sound. In fact, Turks frequently borrow this sound into their language as an E, like in the name "Kerim" for example.

It's Baarid.

baarid

It baarid