Learn Finnish: Minä otan vettä.

Vettä is in the partitive case. Because water is a mass noun (an uncountable substance), we use the partitive when it is the object of the sentence to express an indefinite amount, meaning "some water". If you used the basic dictionary form vesi, it would sound like you are taking all the water in the existence!

Wait, how exactly does the spelling change from vesi to vettä?

Many old Finnish words ending in -si have a tricky stem change. Before adding the partitive ending -tä, the -si changes to -t-. So, vesi turns into the stem vet-, and adding -tä gives us vettä.

The dictionary form of the verb is ottaa. Why does it only have one "t" in otan?

This happens because of a spelling rule called consonant gradation. When you add a personal ending like -n (for "I"), it "closes" the end of the word. This pressure causes the strong double consonant tt in ottaa to weaken into a single t in otan.

Could I drop the word minä and just say "Otan vettä"?