Learn Icelandic: Ef þú lærir ekki, fellur þú.

This is the classic V2 (verb-second) rule in action. The entire first clause (Ef þú lærir ekki) counts as the first chunk of information in the sentence. To make sure the main verb stays in the second position overall, fellur must come immediately after the comma, swapping places with the subject þú.

I thought the verb falla meant "to fall". Why is it translated as "fail"?

You are right that falla literally means "to fall." However, in an academic context—like talking about classes, exams, or grades—Icelandic uses falla to mean "to fail." (If you want to say you passed a test, you would use the verb ná, which normally means "to catch" or "to reach").

English makes a big difference between "learning" and "studying." Does læra cover both?

Yes, it does. While English distinguishes between the action of studying and the result of acquiring knowledge (learning), Icelandic generally uses læra for both. In this sentence, lærir clearly refers to the act of studying.

In English, we might say "you will fail." Why doesn't Icelandic use a future tense here?

Icelandic does not have a dedicated future tense in its verb endings. It relies heavily on the present tense to talk about the future, letting the context (like an "if" condition) make it obvious that the result will happen later.