The correct is "leio". "Lê" is for "ele/ela". And "li" is the past tense of "leio"

Duolingo should accept "li" as a translation of "read", since "read" could be past tense, but they really want the present tense "leio".

I read=Eu leio; you read=tú lês/você lê; he/she reads= ele/ela lê (remember that there's no neutral pronoun in portuguese and we normally use the demonstrative "isto"); we read=nós lemos; you (guys) read= vós ledes/vocês lêem ("vós" is NEVER used in common talks; just early last century's texts uses this pronoun, often referring to only one person as a very polite form); they read= eles/elas lêem.

Remember that even being about the 2nd person, "você(s)" is conjugated as the third person ( just like all the treatment pronouns); the reason is : every time you call someone "você"(archaic form "vossa mercê"=thy/your grace) you're talking WITH the to person but TO his grace; one time his grace is not in the talk, you must use the third person.

Eat pineapples would actually be "come abacaxis." De is used with gostar (and a some other words too, which I cannot think of at the moment). For example: Eu gosto de abacaxis.

As for the explanation of when to use 'de', I would suggest that you friend Davu and send him a message on the Duo site. He had a really good post on this. I wish I could give you the link. He would be able to give you the best explanation. You could also just start a discussion on the site and get some answers. Sorry I couldn't help more.