"Is it cold?"
さむい falls into the category of adjectives known as い-adjectives where い is the stem of the word.
So if you want to say "the weather is cold" you say: てんきがさむい
If you want to say "the weather is not cold" you must change the stem by replacing い with くand then attaching either of the negative forms: てんきがさむくない(です) or てんきがさむくありません
If you want to change the adjective into an adverb use the same process as forming the negative form by using く then add て.
That car is slow: あのくるまわはおそい
That car is not slow: あのくるまはおそくない
That car goes slowly あのくるまはおそくていきます
That car does not go slowly あのくるまはおそくていきません