"Ti presto la mia macchina."
This is more of a general comment really that could apply to all sentences like the one above.
If I was really translating it in to English (as opposed to doing a Duolingo lesson) I wouldn't actually say "I lend you my car". It sounds ridiculous. "I'll lend you my car" is more correct despite being a different tense, (and therefore not a direct translation).
As Italian seems to use the present tense where in English we would use the future, perhaps more advanced versions of duolingo would take these subtleties of language into account, and "I lend you my car" wouldn't be accepted as a correct translation :)
Cosa ne pensate?!