"I am reading important letters."
Translation:Mi legas gravajn leterojn.
"Mi estas leganta gravajn leterojn" I got this as a solution. Where did 'leganta' come from?
it's the Esperanto version of the progressive tense. And in a grammatical context like this, it really should only be used on those rare occasions when you need to emphasise that it is something being done at this exact time, unlike in English where it can also mean near future, or something started in the past that is not yet finished regardless of whether it is done at the very moment of narration.