They all work for me.
Interested in Language
My question is: are the sentences below correct and natural?
1. It has never been easy being a single parent.
2. Being a single parent has never been easy.
3. It isn't easy being a single parent.
4. Being a single parent isn't easy.
5. It has never been easy to be a single parent.
Same here, but 3 & 4 are not the same in meaning as the others as they only refer to the present.
It isn't easy to be a single parent. <- Is it ok, too?
The person is a single parent that means "It is a person who has a dependent child or dependent children and who is widowed, divorced, or unmarried".
I know what a single parent is, but the speaker/writer might be one or not, and that is the kind of factor that might influence which form they used. Your sentences are correct, but they wouldn't necessarily fit every context in an identical manner.
Unfortunatelly I don't quite understand. Maybe any example?