Learn Finnish: Minun pitää siivota keittiö lepäämättä. - I have to clean the kitchen without resting.

The basic verb is levätä (to rest). To form the third infinitive, we use the strong verb stem. You can find this by thinking of the third-person plural: he lepäävät (they rest). Notice the consonant change from v to p. We drop the -vät to get the stem lepää-, and then add the abessive ending -mättä to mean without resting.

Why does the word end in -mättä instead of -matta?

This is because of vowel harmony. Since the stem lepää- contains the front vowel ä (and no back vowels like a, o, or u), the ending must also use the front vowel ä.

Why does the sentence start with Minun pitää instead of Minä pidän?

Pitää is a necessity verb here, meaning must or have to. In Finnish, necessity verbs require the subject to be in the genitive case (ending in -n). Also, the verb pitää doesn't conjugate to match the person; it always stays in this third-person singular form.

Why is the object keittiö in its basic form rather than ending in -n?