1) Only once in a blue moonth does Robert take in playing golf with me as he's overwhelmed by his work. 1) Only once in a blue moon does Robert play golf with me as he's overwhelmed by his work.

3) For once you were mistaken with calculating this math taskproblem. 3) For once you were mistaken with calculating this math problem.

4) Alex, this year you'll be passing your final exam at school, so brush up on the school subjects once in a while. 4) Alex, this year you'll be passing your final exam at school, so brush up on the school subjects once in a while.

5) The young thungs dispersed at once, after hearing a police siren. 5) The young thugs dispersed at once, after hearing a police siren.

5) The young thungs dispersed at once, after hearing a police siren. 5) The young thugs dispersed at once after hearing a police siren.

"," is like and, so you don't need it here.

1) Only once in a blue moon does Robert play golf with me, as he's overwhelmed by his work. 1) Only once in a blue moon does Robert play golf with me, as he's overwhelmed by his work.

3) For once you weremade a mistaken with calculating thisthis type of math problem. 3) For once you made a mistaken with this type of math problem.

It doesn't make sense for someone to sometimes get a specific math problem right and sometimes to get it wrong, so either you should broaden the category or respecify. "To be mistaken" in this sense doesn't work either: with that, you need to specify what the mistake was.

4) Alex, this year you'll be passtaking your final exam at school, so brush up on the school subjects once in a whila bit more. 4) Alex, this year you'll be taking your final exam at school, so brush up on the school subjects a bit more.

we take exams once in a while means pretty rarely