IF I HAD STUDIED HARDER, I WOULD HAVE GOT A BETTER MARK.

Se avessi studiato con più impegno (2), avrei ottenuto un risultato migliore.

IF THEY HAD SLEPT EARLY YESTERDAY, THEY WOULD NOT BE LATE THIS MORNING.
Se avessero dormito presto ieri sera (3), non avrebbero stato (4) (5) ritardo stamattina.

Se ieri fossero andati a letto presto, stamattina non sarebbero in ritardo.

IF Elena HAD NOT CLIMBED THE MOUNTAIN THAT DAY, SHE WOULDN'T HAVE DIED.
Se Elena non fosse salita la montagna quello giorno, non sarebbe morita ora (6).

Se quel giorno Elena non avesse scalato la montagna, non sarebbe morta.

(1) Un po' means a little while harder is comparative.
(2) Se avessi studiato di più is an alternative.
(3) Nothing corresponds to sera in your English sentence.
(4) I don't think that the compound conditional translates your English sentence because it implies that the persons in question have already arrived and they were late.
(5) A similar English clause which translates into a compound conditional is they wouldn't have been late --> non sarebbero arrivati in ritardo.
(6) If you translate wouldn't have died, you're talking about the past and therefore ora can't be used. If instead you want to talk about a hypothetical present condition of the subject, you can write ora non sarebbe morta (here the adverb opens the clause and morta is the nominal part of a present tense), which more or less translates now she wouldn't be dead, but, since the condition of not being dead sounds rather peculiar to me, I would suggest ora sarebbe viva (or, more typically, sarebbe ancora viva) as an alternative.

This is a very difficult topic: you did a pretty good job!  :-)

Esercizio sul  condizionale composto

esercizio di condizionale composto