The prefix for "accustomed" is "ac-" which means "toward" or "to" in this context.
unaccustomed
To add a prefix to the word "definitely," you could use "in-" to form "indefinitely."
"Possibility" doesn't have a prefix. You could add the prefix im- to it to make the word impossibility.
You could add the prefix af- to it to make the word affluent.
endo
You could add the prefix im- to it to make the word impossible.
You could add the prefix ad- to it to make the word advantage.
You could add the prefix sur- to it to make the word surround.
In the word "inform", in- is a prefix. You could add the prefix mis- to it to get misinform.
"Social" doesn't have a prefix. You could add the prefix un- to it to make the word unsocial.
"Patients" doesn't have a prefix. You could add the prefix in- to it to get the word inpatients.