Translation:It is not hot.
Both あつい and さむい are i-adjectives, and when you wanna negate them you remove the い and add くない. Other i-adjectives have their adverb form, which you use to describe verbs, not nouns, and to make the adverb you change い to く, like はやい into はやく ("quick" into "quickly").
But in this case it is the negation form くない. Not あつく, but あつくない.
To describe the weather, あつ=hot while さむ=cold.
To conjugate: +い="it is" while +くない="it isn't". Ex. さむい="it is cold" while さむくない="it isn't cold"
Because they conjugate, you have to put them at the end of the sentence.
You CAN add です even after the adjective to make it more polite. Only conjugate the adjective! Ex. 今日は さむくない です。 is correct but 今日は さむくない てわありません。 is wrong!
Can somebody please explain to a non-native English speaker what the difference between warm and hot is in the context of weather, and why my "its not warm" answer was rejected? An object that is hot will generally be painful to the touch, and a warm object not, but my intuition is that they are the same thing, when it comes to weather.
I would say the same difference applies for weather, like when it's warm it is nice, comfy, but when it's hot it is bad and sweaty. And when it's cool it is refreshing, but when it's cold your hands freeze and you're shaking.
But I think your answer wasn't accepted mainly because there is a word for warm other than あつい and it is あたたかい. They would have used that if they meant warm.
Yes. But one has か in the end to make it a question, while the other is a plain statement.
When we ask for a confirmation in english we put the negation at "isn't", in japanese we put it at the adjective.
あつくないです - it is not hot. (it is not. period.) あつくないですか - isn't it hot? (no? you sure? ok.)
It's something unfortunatelly you'll just have to get used to without trying to translate it, just understanding it. When there is くない plus question, they mean they want confirmation from the listener. You have to know this is the way the sentence is built in japanese. Don't think about how you would build it in english or it will confuse you even more.
DebAzevedo has already explained in detail, but to put it shortly: あつい is the basic form, あつく isn't anything, あつくない is the negative form.
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