Apu tshissenimak nepakue. I do not know whether s/he is asleep.
Tshitshissenimau a nepakue? Do you know if she is sleeping?

This form is used when there is doubt about the event described by the verb. We do not know whether this event is happening or not.

Initial change (or the changed form) is required for the present tense forms in this conjugation when there is no preverb. For example, the dictionary form of nepakue is nipau. The vowel of the first syllable i was changed to e.

Apu tut tshissenimak iakushikue. I did not know whether s/he was sick.

Link to the conjugation guide.