Hi njanja
Using the determiner 'the' assumes the writer
knows what the subject matter is (definitive)
Using 'a' assumes the subject is
not known to the writer, and is more vague in it's usage
They are getting divorced for the second time is fine, as this assumes the writer does have 'inside knowledge' here
In regards to 'a fourth, a fifth time etc', this is acceptable because the writer knows the existence of 'a time' but does not neccessarily have knowledge on the precise details
If they do know, then you would expect this to read:
I've told you not for the fourth, but for the fifth time to stop doing that
In addition, consider this:
The man heard a deafening scream
This would assume the man
did not know the source
The man heard the deafening scream
This would assume the man
did know the source
This is a simple rule you should bear in mind when writing
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