Here are some examples:
* Submarine: A vessel that travels underwater.
* Subterranean: Located beneath the surface of the earth.
* Subordinate: Lower in rank or importance.
* Substandard: Below the usual or acceptable standard.
* Subtropical: Located in a region just below the tropics.
It can also imply a secondary or subordinate aspect, as in:
* Subcommittee: A smaller committee that reports to a larger one.
* Subplot: A secondary story within a larger plot.
* Subspecies: A subdivision of a species.
The meaning of "sub-" can vary slightly depending on the context, so it's important to consider the word as a whole to understand its meaning.