"The coffee is not cold."
Translation:La kafo ne estas malvarma.
Because the negative is formed "ne + verb + whatever comes next", as you'll see as you progress in the course (ne always comes before the verb).
In esperonto the word order for the most part does not matter to much. If you look up the 16 rules you will see why.
So... What would be the difference between saying "La kafo ne estas malvarma." and "La kafo ne estas malvarmo." in this case? Or is the second way simply incorrect? Do the endings of adjectives ever change?
"La kafo ne estas malvarmo" The coffee is not warmth
Edit: Sorry, malvarmo is obviously cold, not warmth. Warmth is varmo. I think you get the point, though. -o makes it a noun while -a makes it an adjective.