In siti mea - Psalm 68:22

Latin Learning Latin
1

Et dederunt in escam meam fel: et in siti mea potaverunt me aceto. And they gave me gall for my food. And in my thirst, they gave me vinegar to drink.

Sitis is a feminine third declension I-stem noun. Why is it spelled siti instead of site?

I also don’t understand why aceto is in the dative or ablative.

2

The ablative of sitis is siti. This is true for other words having pure i-stems.

As for aceto, it’s instrumental ablative. Poto here is causative and the object with which you make somebody drink must be in the ablative.