Learn Swedish: Maten är varm.

This is because of the noun it describes! Mat (food) is an en-word. When a singular en-word is the subject of the sentence, the adjective describing it stays in its base form, which is varm. If we were describing an ett-word (like vatten - water), we would add a -t and say vattnet är varmt.

How does the word maten mean "the food"? Where is the word for "the"?

In Swedish, "the" isn't a separate word that comes before the noun. Instead, it gets attached to the end! Because mat is an en-word, we add the suffix -en to the end of it to make maten (the food).

Does the verb är change if we talk about more than one thing?

No, Swedish verbs are very friendly to learners! The verb är means am, is, or are, and it never changes form based on the subject. So whether you are talking about one thing (maten är) or many things, the verb stays exactly the same.

Are varm and the English word "warm" related?

Yes, they are! English and Swedish belong to the same language family, so they share many words that look and sound similar. Just remember to pronounce the Swedish v like an English "v" (not "w").