gemein
mean
Malicious, unfair, or hurtful toward other people.
Es war gemein von dir, deinem kleinen Bruder das Spielzeug wegzunehmen.
It was mean of you to take the toy away from your little brother.

mean / common
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mean
Malicious, unfair, or hurtful toward other people.
Es war gemein von dir, deinem kleinen Bruder das Spielzeug wegzunehmen.
It was mean of you to take the toy away from your little brother.

In modern everyday language, 'gemein' (mean) almost always means 'malicious,' 'unfair,' or 'hurtful.' It describes negative behavior toward others.
Hör auf, so gemein zu deiner Schwester zu sein!
Stop being so mean to your sister!
'Gemein' (mean/common) is related to 'gemeinsam' (together/shared) and 'die Gemeinschaft' (community). The original meaning was 'belonging to everyone.' From that, the modern negative meaning developed through 'ordinary' and 'vulgar.'
Früher bezeichnete man die einfachen Bürger als das „gemeine Volk“.
In the past, ordinary citizens were referred to as the 'gemeine Volk' (common people).
The old meaning 'gemeinsam' (shared/common) has been preserved in the fixed expression 'etwas gemein haben' (to have something in common). Here the word has no negative meaning.
Trotz ihrer Unterschiede haben die beiden vieles gemein.
Despite their differences, the two have a lot in common.
simple / just
'einfach' and 'gemein' can only be close in meaning in an old sense: 'gemein' used to be able to mean 'simple' or 'ordinary'. Today, however, this meaning is outdated. In modern German, 'gemein' almost always means unfair, nasty, or hurtful. For that reason, confusing the two can be problematic. If you want to say that something is plain or without luxury, use 'einfach'. Today, 'gemein' is used for unfair behavior or a hurtful action.

Es war gemein, dass Max den anderen Kindern nichts von dem Spiel erzählt hat.
It was mean that Max did not tell the other children anything about the game.
Im Urlaub wohnen wir in einem einfachen Zimmer ohne Balkon.
On vacation, we are staying in a simple room without a balcony.