under

understand, undertake, undergo, underway

prefix under- (too little, below, under):
underneath, underside, underlie, underscore, underline, undersign, underwrite, underpin, undersea, underwater, underground, undercurrent, underflow, undertow, undercover, understate, understatement, underpromise, underestimate, underrate, underdeveloped, undertone, underarm, underbelly, underfoot, underhand, undermine, undercut, underbid, underperform, underreact, underwhelmed, underemployed, underpaid, underpay, underdog, underclass, undernourish, underprovision, underprivileged, underserved, underappreciated, undermotivated, undercompensate, understudy, undertrained, undermount, undersink, underclothes, undershorts, underwear, undershirt, underdress, undercoat, undersize, underage, underweight, undergrown, underfeed, undercook, underbake, underdo, underdone, underachieve, undercharge, underprice, undersell, undersold, undervalue, underbuy, underentitlement, underbrush, underexpose, underemphasize, underutilize, underuse, underrepresent, undercapitalize, underfund, underinvest, underinsure, underprotected, understaff, underreport, undercommunicate, undercount, undersupply, underdeliver, underpower, underinflated, underfill, undertreat, underbeat, undermix, underpaint, undercolor, understory, undergird, undercarriage, underlet, undersecretary, undergraduate (undergrad)