Translation:Our father used to be in the army.
How can 'used to' be used with a telic verb like 'join'? This is uninterpretable in English (unless the father joined, quit, joined, quit, joined, […]), so I assume 'từng' has a broader semantic field than English 'used to', although I can't imagine what the sentence could mean.
I just noticed you never got an answer to your question about "từng". You're right that the meaning is broader than the English "used to". In fact, its meaning might be most precisely intelligible as the "once" of "we were once friends," or "once upon a time". Naturally, this is distinct from the "once" that means "exactly one occurrence". So Duolingo's sentence here might be best translated as, "Our father once joined the army." This also helps clarify the construction "chưa từng" meaning "have not ever". For example, "Tôi chưa từng thấy nó" ("I haven't ever seen it," or more literally, "I am yet to see it -- ever").
Would this sentence be okay: Tôi từng đi học mỗi ngày (I used to go to school everyday)
I reckon 'từng' can also be used (sorry!, completely unintentional usage of 'used') to ask the question 'have you ever.......[yet]?.'. e.g. Bạn đã từng đến Việt Nam chưa? (Have you ever been to Vietnam [yet]?)
Then there is 'thường' which can be used to say 'often/usually'. Also: thỉng thoảng= 'sometimes'. Luôn luôn mean 'always'. không bao giờ = never. chưa bao giờ = not yet.... ít khi = rarely
Have I left anything out?
There are a good number of sentences in the course that use ungrammatical/awkward English, and most seem to be due to hyperliteral translations (I feel painful, the bird lodges at the branch, etc). Are most of these because of the constraints of duolingo itself and the hint system or is it a design choice by the team?
Just curious because I got my tendency to do "translationese" squashed at school, so it's taking a little getting used to : ) Though I notice less awkward English at this stage of the course than I did at the beginning. Could just be me, though.