Learn Italian: L'acqua è quasi troppo calda. - The water is almost too hot.

In this sentence, troppo is acting as an adverb meaning "too," because it describes the adjective calda (hot). Adverbs never change their endings. If you wanted to say "too much water," troppo would be an adjective describing the noun itself, and then it would change to match it: troppa acqua.

Wait, doesn't calda mean "cold"?

It looks a lot like the English word "cold," but it's a false friend! Caldo (or calda) actually means "hot" or "warm." If you want to say the water is cold, you would use fredda.

Why does calda end in -a here?

Because it describes l'acqua (the water). Acqua is a feminine singular noun, so the adjective describing it must also be feminine and singular. Therefore, caldo changes to calda.

Why is it written as l'acqua instead of la acqua?

Since acqua is a feminine noun, the word for "the" is la. However, because acqua starts with a vowel, the a in la is dropped and replaced with an apostrophe to make it flow better when spoken. This is very common in Italian before words starting with vowels.