"They read her a magazine."
Translation:Eles leem uma revista para ela.
Oi Carol, eu não concordo. Quite on the contrary, I'd have to agree with G.P.Niers, even though without sound (which I don't have, unfortunately), English 'read' could also be presente indicativo, 3ra pessoa/pl. Your suggested sentence "Eles leram a revista" does not fit the English sentence for two reasons;
You are using the specific article "a", which would translate to English 'the'. The exercise says "a magazine".
The exercise contains an indirect objective, "her" (to whom are they reading) which you fail to include in your PT sentence.
My suggested translation failed because I attempted using 'a' rather than 'lhe', exactly as G.P.Niers points out above, i.e. the difference between direct and indirect objectives. :-)