Translation:I exercise twice a week.
In this context (doing physical movment) "exercise" and "work out" are often used interchangeably. 运动 has to do with motion while 锻炼 has to do with strengthening/ training, so "work out" is probably best covered by 锻炼 but that doesn't mean it shouldn't be accepted here.
So what is the difference between "运动" and "锻炼" ? Both have meant exercise. I understand "锻炼" is also to work out, but it also meant exercise.
Is the first word a noun and the second one a verb? Why don't we need to place a "的" before the activity. Is it not necessary if we use "次" ?
行使 is more like "exercise ones power/ ability" or "perform/ show what you can do"
"Every week I do two exercises" was marked wrong. How do you know when they mean ''exercise'' and when they mean "sports"?
I think the confusion is that “exercise” in English means either “to exercise” or “an exercise” ...but perhaps 运动 doesn‘t?
Is there another word for ‘an exercise’ in Chinese? Meaning a particular exercise like a push up or a squat, or a single form in tai chi etc?
I wrote: "every week i do sport twice". I mean, can't you just accept it?
Your sentence is ungrammatical. They do accept the grammatically correct answer "Every week, I do sports twice." based on a suggested answer I was given in response to my wrong answer "Every week, I do two exercises." though.
twice a week isnt great grammar is it? shouldnt it be twice each or twice per?
I think "twice a week" is fine. It is not formal, but it is not incorrect. Here is a discussion on this topic: https://english.stackexchange.com/questions/62009/difference-between-per-and-a.
I agree that "twice a week" is fine. They mean essentially the same thing (i.e. "per" and "a"), but I also liked the post in the link that said using "per" can sometimes imply a more formal or slightly stricter adherence to a routine, whereas using "a" allows that the frequency can be a little more loosely or informally adhered to. However this is just a question of common usage and can vary according to context.