"The men are good."
In Hebrew "אנשים" refers to people in general, both men and women (even though singular איש is only male). For a group consisting of males only, "גברים" is more accurate.
So now I'm really confused. I was told that the difference between "a good girl" and "the girl is good" is if the suffix matches. But here the suffix matches so I wrote "the good men" and the answer was "the men are good". How do I figure this out on my own?
I'm not sure what you were told, but this is not true: the adjective must match the noun.
a good girl = ילדה טובה
the girl is good = הילדה טובה
the good girl = הילדה הטובה
and similarly for this sentence -
good men = גברים טובים
the men are good = הגברים טובים
the good men = הגברים הטובים
And there would only be et before if there was a verb before? Why isn't it lamed instead of hey? Thanks in advance
There's no ל in this sentence.
ll את comes before a direct definite object (and this sentence has no object). Usually, that means you'll find את in sentences with verbs (but also with יש, אין and צריך).