When do you use singular or plural, as between skill vs skills, relationship vs relationships, floodwater vs floodwaters, responsibility vs responsibilities?
For example:
I don’t have the required skill(s) to do the job.
He does not have good relationship(s) with many people.
Language skill(s).
It is our responsibility(ies) to help other people in need.
In the near future, I hope I have a chance
to develop my interest and capability here.
(interest and capacity should be plural?)
You have to follow the logic of whether you are speaking about one of those things or more than one of those things. Both are possible in many of them so you need to look at the wider context.
Both are possible. It depends on whether you are speaking about one overall skill or many smaller skills.
Both are possible, though an article would be required with the singular form. Again it depends on whether you are speaking about his (singular) relationship with people in general, or the (plural) individual relationships he has with many people.
Both are possible, depending on context.
This should be ‘responsibility’ in your sentence. Here’s an example where both are possible:
We have responsibilities towards people in need./ We have a responsibility towards people in need.