sometime = at an unspecified time. I'll see you sometime next week. some time = a significant amount of time. I'll need some time to complete my essay.
There is also:
sometimes = now and then; occasionally. Sometimes I go to the gravel pit and roll around in the sand.
The single word "sometime" means "at an unstated or unkown point in time".
In the collocation "some time", you basically have the word "time", and it is modified by the word "some" in the usual way. If you say "I have some time for a short meeting tomorrow afternoon" that basically means you have "an amount of time" availabl