Learn Russian: Эта оценка гораздо важнее, потому что это экзамен. - This grade is much more important, because this is an exam.

It is the comparative form of the adjective важный (important). To make it, you drop the standard adjective ending and add -ее. A great thing about these -ее comparatives is that they never change to match gender, case, or number!

Could I use очень (very) instead of гораздо here?

No, just like in English, you wouldn't say "very more important." In Russian, you must use гораздо (much) to make a comparative word like важнее stronger. You use очень only with regular, non-comparative adjectives, like очень важная (very important).

Can I say важней instead of важнее?

Yes! For most comparatives ending in -ее, you can shorten the ending to -ей. Важней means the exact same thing as важнее. The -ей version is just a bit more casual and very common in everyday spoken Russian.

Why is the first "this" translated as Эта but the second one is это?