なる
when combined with なる, implies you stop doing something habitually
肉を食べなくなりました。 I stopped eating meat.
with 〜て
Combining the negative form with 〜て has two options:
ないで
ないで is more common to express "without doing/ not doing"
まだプレセントをあげないでね。 please don't give him the present yet.
なくて
なくて is more common to express reason/cause
もらったクッキーを弟にあげなくて、妹にあげた。 I did not give the cookie to my brother, I gave it to my sister (implies a reason for not giving it to brother).
Abbreviating to ん
Often abbreviated to just ん in some dialects.
韓国人と結婚してはならん！ You must not marry a Korean
すまん Sorry! (from すみません)
そんなことはさせん I won't let you do such a thing (using Causative form (〜aせる))