It rejected "they study only one hour per week". This is the correct position for "only". Reported 23 April 2018.
That's interesting. To me neither one sounds as though it suggests anything, but at a push I'd say 'they study only...' sounds more like a suggestion that they should study more, because it's next to the time (one hour) and therefore sounds more like the time is being emphasised. It's intriguing to me how different people interpret things like this.
Same here. Reported 24th September 2018. I think it is PER they have the problem with, they want A week.
You are correct. Duo wants "a" instead of "per," which is ridiculous because "per" is more accurate.
Here is another common mistake by Duo in the placement of only in this sentence. If the meaning to be conveyed is that it is 'they' and nobody else that studies, or that it is 'study and not something else as well that they do, then the only is in the correct position.
If, as I suspect, the meaning is that their study period is one hour per week and no more, then the only should be placed after 'They study' and before 'an hour...'.
It may be common practice to place the only in this position, but that doesn't make it good grammar.
If it is 'they' and nobody else that studies, I think it would be more common to say 'Only they study and hour a week'. That way the 'only', only modifies them and no other parts of the sentence.
'They only study an hour a week' and 'they study only a hour a week' sound like they both would be commonly used to refer to their bad study skills. It may be a regional thing, but both sound good to me.
If it is study, and nothing else that they do for an hour, I'd probably throw in more words or switch them around to specify that. Something like: 'For an hour a week, they only study.' 'They study, and do nothing else, for an hour a week'.
*I'm not claiming any of what I just replied is proper english, just that it would most likely be understood in everyday American english.