"The subjects that he chose were too hard."

Translation: Předměty, které si vybral, byly příliš těžké.

[Praha2017]

We were taught the relative clauses with "jenž ...", so now their usage should be accepted. ;-)

[VladaFu]

They should, but be aware that almost no-one uses "jenž" and derivatives in speech. I don't use it even in writing. It is just too formal or bookish or old... So, it may be forgotten as a possibility here and there.

[VladaFu]

Moreover, this is quite far in the course and there will be more missing translations than in the first lessons. This particular sentence is missing many possibilities.

[strubkin]

Why I cannot use "on" here? "On si vybral"

[VladaFu]

Second position rule for si. Most likely, that is. You should report the complete sentence.