Sobre a omissão da palavra QUE

I use the site Memrise to learn new words in English. There have a message:

Review words you've learned.

The translation for our language is:

Revise palavras que você aprendeu.

Why we do not use the word que in English?

Thanks a lot!
MENSAGEM PATROCINADA Faça um teste de inglês e descubra seu nível em 15 minutos! Este teste foi desenvolvido por professores e linguistas certificados. O resultado sai na hora e com gabarito.

Clique aqui para iniciar o Teste Online!
Avatar do usuário PPAULO 39785 6 32 695
You somehow was misled by the example.
In fact it´s entirely possible to write (and to find elsewhere) the sentence "review words that you have learned."

Thus, in our case here the ommission of the relative pronoun "THAT" is possible because it is the very " object of the relative clause".
In this case the process is similar to what happens with pronouns! They can replace the noun/the subject as well is of that that I am talking about, please don´t say I claimed that they can be left out...!

Summing up, in "review words that you have learned.":
THAT refers to WORDS; but then you had used WORDS then, so THAT could be left out with no loss of meaning.

As a rule of thumb, when we leave out the relative the "that" (here in our example) and the sentence don´t get "weird", when we understand it just the same, then we can ommit the relative pronoun. I mean, with time we get used to it, we do it even without thinking of the grammar note.
But that comes with time, the experience, for the time being stick to the rules.