"We didn't read the book"
That would be the infix -ku-. This affix has multiple meanings depending on the context, and here it is a tense affix.
Tense infixes are different in the affirmative and negative, (for affirmative tense infixes refer to the Tense post in the Masterpost)
In the negative:
- Past: -ku- e.g. 'Sikulala (I did not sleep)
- Present: -i e.g. Silali (I am not sleeping)
- Future: -ta- e.g. Sitalala (I will not sleep)
Note that the present tense affix is actually a suffix, and the future tense infix is the same in the affirmative and the negative