There aren't any other gods.
There you go! I knew you could figure out the grammar.
But monotheism is interesting, because it's kind of the worst of both worlds:
- on the one hand, it has all the logical problems of theism in general.
- on the other hand, except for the one God that the person believes in, it has all the logical of the strongest of strong atheism (i.e. claiming with certainty that no other gods exist).
... and your hands are a bit tied on that second point, since the typical strong atheist arguments for the non-existence of any gods at all aren't available to you, since they'd violate the first point.
Monotheism is pretty logically tenuous. I've never seen anyone give a rational defense of it; I've never seen someone give a reasonable argument for why one god exists AND no others exist, aside from theists who define "God" as something that there's apparently only one of (e.g. the "God is the universe" of pantheism).
When you think about it, monotheism is way more extreme about the non-existence of gods than even the most ardent atheists. Most atheists' position is something like "the evidence for gods is unconvincing, and the time to believe is when we have convincing evidence." You and your fellow monotheists go several steps further; you guys say, effectively "no, no - not only are we unconvinced of those other gods; we KNOW that our God is the only god that exists."
Like I said: it's the worst of both worlds. It has all the problems of theism in general combined with all the problems of strong atheism (and then some).