"She sells dresses."

Translation: Dia menjual gaun.

[cornishpixies]

Why not gaun-gaun?

[Green_wood]

Why not gaun-gaun? Isn't it a plural?

[deactivated user]

Whats the difference between 'mem' and 'men' before words? Something about past tense?

[Rick392366]

No, it's not about past tense, no need to worry about tenses, Indonesian doesn't use tenses.
"me-" prefix converts a base word into a verb.

It depends on the starting letter of the base word if "me-" is 'conjugated'.
The conjugation table looks like this :

Starting letter base word	Conjugation
l, m, n, r, w	Me + base
vowel, g, h	Me + ng
c, d, j	Me + n
b, f, v	Me + m
k	Me + ng – k
t	Me + n – t
s	Me + ny – s
p	Me + m – p
monosyllabic	Me + nge +base

Here is a Tinycards deck with more examples :
https://tinycards.duolingo.com/decks/7XE3GxCr/me-prefix-bahasa-indonesia

No need to do the lessons there, just flip through the cards of the deck to see how a base word is converted into a "me-" verb.