Learn Finnish: Älä revi sivua. Älä taita sitä. - Do not tear the page. Do not fold it.

In negative commands, you use älä followed by the verb's connegative stem. To find this stem, take the present tense minä (I) form of the verb and drop the final -n. For example, repiä (to tear) becomes minä revin, so the stem is revi. taittaa (to fold) becomes minä taitan, giving the stem taita.

Why do the consonants change from p to v and tt to t in these verbs?

Both repiä and taittaa are Type 1 verbs that undergo consonant gradation. Because you find the command stem by using the minä form, the stem takes the weak grade. This softens the p to v (repiä to revi) and the double tt to a single t (taittaa to taita).

Why do both sentences use the partitive case for their objects (sivua and sitä)?

A major rule in Finnish is that the object of a negative sentence must always be in the partitive case. Because you are telling someone not to do something using älä, the base words sivu (page) and se (it) must become sivua and sitä.

Wait, is sitä just the partitive of se?