"The men eat papaya."
Translation:Những người đàn ông ăn đu đủ.
You can not use "người" in front of "đàn ông" and it's fine also.
It's just that "người" it's used less used for things like "cô gái" even though it's formally correct.
Mainly because the older the people the more respect should be paid to them. As Vietnam's culture is like that. So you see more formal language when speaking about people that are older than you.
Front-mouth vowels o, ô and u cause the lips to close with the ng or c, producing a co-articulation with the tongue in ng position. Basically say ng /ŋ/ or c /k/ and close your lips at the same time. i.e. -ong -ông -ung -oc -ôc -uc. I think that's all. (Obviously -p as well but it's a p so no co-articulation.) I have seen some accents close their lips for -t too, e.g. tốt, but that was when I was new to Vietnam and probably I didn't understand what they were teaching me.
Back-mouth vowels ư and ơ do not cause this to happen. The same is true for the other vowels.