"They learn an English word."
Translation:Ili lernas anglan vorton.
The first one. The sentence should be "Ne, li diras, ke tio estas la traduko."
As for why, having two verbs of the same tense together tends to be considered poor grammar in most languages. If you are trying to say Is saying in Esperanto, then the ~as affix denoting the present tense makes the "is" for you. Otherwise you are are saying "He is is saying…"
In Esperanto the position of the noun and adjective is actually immaterial. This course seems to be maintaining a more rigidly English standard of adjective first, but, per Zamenhof and many others, the exact order is left flexible so that native speakers of other languages may speak in a way more comfortable to them. It also allows for very flexible sentence structure which is an incredible boon to poets, writers of aphorisms, and anybody else who likes to play with where might lie, of a sentence, the emphasis.
"An" is not expressed by any word here - neither is it expressed by part of a word. Mostly it's context that tells you that it's "an." The sentence can be translated word-for-word as "they learn English word" -- and any English speaker would know that this needs "an" to be grammatical.
"Oni" is the "they" in "that's what they say"... if you can say swap out "people" for "they" then it's probably "oni."
It's not a tricky concept, but it can be confusing because we don't have one word for in English.
- You don't tug on Superman's cape.
- One doesn't just walk into Mordo.
- That's what they say.
All would be "oni."