Not wanting to add to your troubles but in one of my textbooks (Latin Sentence and Idiom, R Colebourn) I found a situation where it is permitted to use future participles as the predicate of a sentence. It appears that, used in that fashion, they mean
likely to or
intending to:
Hostes bellum illaturi erant
The enemy were likely to wage war
Facite quod vobis libet; daturus non sum amplius
Do what you want, I do not intend to give any more.
Interesting, yes? Of course none of this means that you can say
sum necans to mean "I am killing" (which is neco).
That helps a lot. I've been reading over chapters 23-25 for a while now. It's like hitting a wall after all going so smoothly.
You'll get it eventually. If you look back at the earlier chapters you'll see that the stuff that seemed so hard then is child's play now. Eventually, you'll look upon Wheellock as a "elementary" work, and you'll be able to pontificate to newcomers on this website on all points of Latin grammar without needing to look anything up.
Have you considered taking a break and reading something (Ovid or Martial are good)? Or are you doing this for schoolwork?