Please, could you clear up for me what is the difference in the usage of the following phrases:
once/twice... per year, once/twice... a year, once/twice.... in a year? Is it true that the first one sounds more official? Is the last one correct at all?

Thank you!
Olga

 

Once a year is also fine and as you suggest, slightly less formal.

 

In every case I can think of 'once a year' is all that's necessary.

Rover

 

I would agree, on the whole; legal documents (which would avoid 'once a year') tend to use 'annually', don't they?

 

I was faced with this problem a while ago, is once in a while wrong ? Or is it the same as once a while ?
Thank you.

 

I've never heard of "once a while". Do you have an example of its use that you can share with us? The correct form, to my knowledge, is "once in a while".

Edited to add: I'm astounded to have just found around 300 uses of "once a while" on Australian web pages. Some are quoting foreign pages and the rest are either very casual language or "(I met him) once(,) a while (ago)", but it looks as though it is used by some sections of the Australian community, in a very informal register, to mean "once in a while". Personally, I find it very odd indeed.

 

Thank you so much for your answer, what about once in a decade/century/year/..., are they equivalent to once a decade/century/year/... ?
This phenomena happens once a decade or this phenomena happens once in a decade ?

 

I think the versions are used in different contexts. Once a decade describes the frequency of foreseeable events; once in a decade describes the frequency of unforeseeable events.

This light phenomenon happens once a decade.
You can feel that happy only once in a decade.

 

Thank you so much.