"That little hook on the e" is called affectionately "ogonek" - "a little tail".
When you talk about the letter "ę", you call it "e z ogonkiem" - "e with a little tail".
It describes the sound which is more or less nasal, but still a bit different from the regular "e".
Its presence may distinguish the verb in the 1st person: "piję", "piszę" from the one in the 3rd: "pije", "pisze", and may indicate the correct case of the noun.
As a countable noun, the word "tomato" requires an article.
The nouns "cheese" and "soup" are uncountable (no article).
The noun "fish" is "plural", so you cannot use "a" (it means 1)
Pomidor i ser - A tomato/ The tomato and cheese
Ryba/Ryby i zupa - Fish and soup/ The fish and soup
Actually fish can mean one single fish, it's not necessarily plural. If you see a fish in a pond you'll say: "look at the fish!" And if there are many you'll say: "look at the fishes!"
So there's a difference between countable and mass noun, similar to fruit / fruits.