Common words:
* understand
* underneath
* underwear
* underground
* underestimate
* undergraduate
* underdeveloped
* underwater
* underlying
* undertake
* undergo
Less common words:
* undermine
* undercut
* underfoot
* underhand
* underplay
* undernourished
* understaffed
* undercurrent
* underdog
* undersecretary
This list isn't exhaustive, but it gives you a good starting point for understanding how the prefix "under" is used in English.