"Moje sestra se bojí myší."

Translation: My sister is afraid of mice.

[snikta]

I was marked wrong for putting frightened but in English it has the exact same meaning as afraid and scared - they should all be accepted

[VladaFu]

I am not the right person to judge that and I will be glad for further comments from English natives, but I think frightened is more about fear that happens just now, not a regular one - similar to terrified.

I have now found this example:

He is a stern master. The boys are all afraid of / scared of him. (More natural than ‘The boys are all frightened of him.’) https://www.englishgrammar.org/afraid-frightened-scared/

[BoneheadBass]

I would agree that "afraid of" or "scared of" would be used a LOT more often than "frightened of," at least in the US.

[JeanneJACQ2]

As a native American English speaker I would say in some scary circumstance, a person would be frightened, ie. I am frightened! But if one is referring to a specific frightful thing, one would say I am afraid of the dark, or I am afraid of spiders, or I fear the dark.

[Janmunroe]

Mysi is genitive, correct? So, which feminine noun paradigm does it follow? Ovce? Or is it another "irregular"?

[svrsheque]

you may just want to memorize the myš declensions. it is not so much irregular as one of a group of unstable nouns that even the plural "děti" belongs to.

[Janmunroe]

Dekuji! Never a dull moment with this stuff!