"We know both men."
Translation:Znamy obu mężczyzn.
It depends on gender and if a noun describes a person, and also on case.
obaj is for male person nouns
obie is for female nouns
oba is for neuter and male nouns that do not describe people
oboje is for:
- neuter nouns that describe people (dziecko=child, niemowlę=baby) ,
- pairs of people or person + animal that have different gender/sex
- pairs of nouns that are "always plural"
obojga is a form of oboje but in genitive.
obu is a form of obaj in accusative, and a form of obaj/obie/oba in genitive.