Is there a term for when you are using “ます／です” vs “plain form”?

行きます
行きません

vs

行く
行かない

When speaking, I often mix up using polite standard “ます／です” form and casual “plain form”.

Ex. 日本語を勉強します。　けどクラス取らない

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Plain form is also used for writing such as novels.

If you add “よ” or “ね” after verb and adjective, it becomes a more natural phrase for your family and friends.

かいものに行くよ
私は行かないよ

Ex. 日本語を勉強します。けどクラスは取りません
It’s more natural and polite.

日本語は勉強するけど、クラスは取らないよ
↑You can use it with your friends.

Sometimes it’s ok to mix up using.