The "accusative case", explained in the adjectives section for anyone looking for this answer
If ryba is feminine, then why does it end with ę here? I'm confused as to when to end a word with ę instead of e, or ą instead of a.
E or ę, and a or ą is based on the noun case. The word by itself ends in a (ryba) but if it is the object of the verb, it ends in ę (rybę). At least that is what Duolingo says is true in this case. Duolingo doesn't explicitly teach this idea, you have to go elswhere. Search for 'noun case' and 'declension'.
I think the confusion comes out as "ta" gets declined as "tę" while "tamta" as "tamtą". 99% of feminine adjectives are declined (in accusative form) as "-ą", while feminine nouns as "-ę". "Tę" (being an adjective) can therefore considered as an exception (I hope moderators will confirm what I'm saying). Powodzenia!
Apart from the fact that "tę" is a pronoun and not an adjective - yes, that's right. But I wouldn't really call anything here an exception.
Why is it "Tamta ribę" but "te wodę" when both words end with "-a" in their normal form?
The Accusative form of 'ta' is 'tę' (which is logical, as it's tę rybę, tę wodę etc.), but there's just no such word as 'tamtę', it's 'tamtą'. Less logical, but it is how it is.
Tę is a relic from 19th century. At that time it was normal to say: tę, tamtę, moję etc. Since then tamtę became tamtą, moję became moją and tę tries to do the same.
Words ending in -a are feminine, words ending in consonants are masculine, and words ending in -o are neuter.
Thank you so much for this info, I don't recall seeing it anywhere else. Have a lingot!