Because the English language has got more than one word. :-) In order to use the definite article the item has to be introduced before in the context.
Ili lernas anglan vorton. – They learn an English word.
‘Potato’ estas ‘terpomo’. Ili lernas la anglan vorton. – ‘Potato’ is ‘terpomo.’ They learn the English word.
That only works when you're refering to the language as a noun (la angla - English) or if your defining the noun that comes with the adjective (la anglan vorton - the English word).
As an English speaker, I know we have the "present imperfect tense" which can also make this confusing. Is there a similar tense in Esperanto? Pardonu, mi estas komencanto! LOL
It also says it can be translated to "They are learning an English word." Does Esperanto have a present progressive tense?
Because it's like that. Esperanto doesn't make a difference between present simple and present progressive like English does. This form is rather to be used as an adjective, for examle “la lernanta knabo estas bela”.