singular form or prural form?

[caronmi]

Dear teachers, I am always confused about how the singular and prural forms vary in some certain cases as follows:

....They also point out that alternative punishments, such as extremely long prison terms, are a great expense to society and rarely result in successful rehabilitation, so that the criminal (or should it be "criminals"?) would be a potential danger to the community.

Can we view "punishments", "terms", or "criminals" as a whole, and then use "a/an" behind them?

I will be most grateful if anyone can kindly explain it to me, or correct my sentence. thank you!

[Tdol]

Punishments- plural, with a plural verb are You cannot use a terms/punishments
The criminal/criminals- either could be used here

would be a potential danger- how about would remain...