Caballo vieja hembra. Or yegua vieja, old mare.
However, nag does not imply female; it applies to stallions too. In the most general sense, it just means horse; the more common meaning is an old or overworked horse.
A quick look on spanishdict.com shows el jamelgo, el rocín, and el jaco (all masculine) as words for nags. (Although, jaco show "small horse; young horse; nag; hack", which goes against the meaning of an old horse, but goes with the concept of a horse not fit for work.)
It is listed as a Carribbean expression. "dar la lata algn" meaning "to condemn or censure someone"
"dar lata" is listed from Andes "To nag, go on" http://dictionary.reverso.net/spanish-english/dara%20la%20lata/forced
Segun el RAE, "Apreder" = "to acquire knowledge".
One meaning of "apprehend" is "to grasp knowledge."
"Understand" is a deeper knowing.
IN Spanish, understand is "entender" or comprender." So, I don't think DL is being nit-picky.
If one want truly to understand a language, one needs to nit pick. Otherwise, the "learning" is superficial, not deep.
The "ir-future" is closest to the English future progressive tense(As they called it when I went to school, no clue what teachers call it now.) Which would make it "When are we going to learn." So you are right about closer/near future(going +infinitive) vs distant future(will +participle.) However, in this case, "When will you learn?" And "When are you going to learn?" they are fairly synonymous. I used the progressive future though because this lesson is about the "Ir-future."