Translation:Men are fast.
I believe using 들 for plural is optional. 남자는 could mean both "man" and "men".
Correct, the plural marker is only used for limited cases such as addressing a group or emphasizing the count
It uses the topic particle ~은/는, which is often used for general statements or statements with respect to the topic. E.g. you would read this as "men, for one, are fast" or "a man, for one, is fast". The topic is men or a man in general.
As for ~들, it's an optional particle that clarifies plurality, otherwise whether or not something is plural is unspecified. In general statements like these they're usually not there because they're unnecessary.
Fun fact, plurality is also unnecessary in English to make general statements about topics. E.g. Instead of saying "rabbits are animals" you'd use a a singular subject and say "a rabbit is an animal", which is still generalizing all rabbits.
Wait I'm still confused on the difference between the two types of subject markers (는/은 and 이/가). Can someone explain?
는/은 means more "a" or, in this case, is used for generally true statements, such as "Men are fast (남자는 빠릅니다)" or "People are clever (사람은 영리합니다)," whereas 이/가 is used as "the" in the sense that it specifies a specific thing, for example, "개가 행복합니다" means "The (this specific) dog is happy (but maybe not all dogs are)."
Correct me if I'm wrong. Used for nouns and pronouns: If the last letter is a consonant, use 은 and 가. Whileas, if it's a vowel, use 는 and 이.
If it ends with a consonant you use 은 and 이 if it ends with a vowel you use 는 and 가