Why is it necessary to add く in the following なる sentence?


This is the full sentence:

> お酒を飲みすぎると、気持ち悪くなる。

なる here means "to reach a certain state." The rest is easy to translate.

The only thing that confuses me is く. What does it mean here and why is it
necessary?




You have to learn how to conjugate i-adjectives when it comes to expressing
change using なる. I recently answered a similar question, but that was posed in
a different way so let me just get to the bottomline of what you want to know.

The basic construction rule when expressing change with なる is the following.:

> い-adj （－い＋く）＋なる Example たかい >>> たかくなる
>
> な-adj （－な＋に）＋なる Example きれいな >>> きれいになる
>
> nouns （＋に）＋なる Example せんせい >>> せんせいになる

And when it comes to negative:

> い-adj （－い＋く）＋ならない Example たかい >>> たかくならない
>
> な-adj （－な＋に）＋ならない Example きれいな >>> きれいにならない
>
> nouns （＋に）＋ならない Example せんせい >>> せんせいにならない

In your case 悪い is an i-adjective, hence 悪くなる.

