"This is a tasty fish."
Translation:To jest smaczna ryba.
"ryba" is feminine, so shouldn't we use "ta" instead of "to" (neuter) or "ten" (masculine)?
"To" isn't modifying "ryba" in this sentence! It doesn't say "this fish" (though that could be implied). You're not picking this fish from among other fish, you're picking this thing from among other things (and saying that it's a tasty fish).
If you were to say "this thing" in Polish, it would be "ta rzecz," not because "ryba" is feminine, but because "rzecz" is. But as in English, you're allowed to use what linguists call a "zero mark" or "∅" here (i.e., leave out a normally expected noun).
Imagine for the sake of illustration that "∅" were a neuter Polish word meaning "thing." To agree with the gender of ∅, you'd say "to ∅ jest smaćna ryba."
Think of it the same way. The zero noun here happens to be formally neuter, so no matter the gender of the other thing the "jest" attaches to, you'll still say "to jest."
Putting it simple: when the sentence would translate as "this is", you always use "to" regardless of gender. Or to put it in another way, when all you do with a fish is to point at it/name it, rather than making it a part of another action.
It is correct on its own, but it's not a translation. The subject here is "This" (a dummy pronoun), you changed it to "This fish".
This is one of our most important rules in this course - don't change the subject of the sentence.