Learn Spanish (Latin America): ¿Lo estás leyendo?

In Spanish, object pronouns like lo (which means "it" here) usually go right before the conjugated verb (estás). But when you use the present continuous (estar plus an "-ing" word like leyendo), you have two choices! You can put lo before the first verb (Lo estás leyendo) or attach it to the very end of the second verb (Estás leyéndolo). Both are perfectly correct and mean the exact same thing.

Why is the word for "reading" spelled leyendo and not "leiendo"?

The base verb for "to read" is leer. Normally, to make the "-ing" form (the gerund) for an "-er" verb, you take off the ending and add -iendo. But if we did that here, we would get "leiendo", which puts three vowels in a row (e-i-e). Spanish changes that middle unstressed "i" to a "y" to make it easier to say, giving us leyendo.

How do we know this means "Are you reading it" if the word tú isn't there?

In Spanish, the ending of the verb usually tells you who is doing the action. The verb estás is the "you" (informal) form of estar (to be). Because estás already tells us who the subject is, adding tú is completely optional.

Could I just say ¿Lo lees? instead of ¿Lo estás leyendo?