This answer is not English!
Duolingo question: Write this in English?: Ich musste Englisch
My answer: I must learn English.
Duolingo answer: You used the wrong word. I'd to learn English.
Has Duolingo content not been reviewed properly? Unfortunately, their answer is meaningless.
I guess the German sentence is "Ich musste Englisch lernen." "Musste" is past tense, therefore it is translated with "had to". Ich muss Englisch lernen. (I have to learn English) Ich musste Englisch lernen. (I had to learn English.)
There is a bug with auto-contractions on Duo, "is" get's contracted to apostrophe-s and "had" to apostrophe-d, even if it doesn't sound natual. This is what happened in your case.
"I had to learn English" is the correct translation of "Ich musste English lernen", like Kartoffeleis said.
I had to learn English not I must learn English
all you must do is formulate the sentences correctly. also, sometimes the translation is not the only answer