Commons lashed in with a 25-yard shot just before the break that fooled Richard Lee and doubled the lead by tapping in Darryl Powell's cross.
Watford should have been ahead but Jay Demerit shot wide and Johnnie Jackson headed off target from a good position.
Forest dominated after half-time, Scott Dobie slicing over and Commons heading at Lee as they could have scored more.
Forest manager Gary Megson:
"We have started recording clean sheets which means we don't have to score three goals to win games.
"This was always going to be a hard week with three away games as we hadn't won away from home all season.
"But when we were nine points adrift we knew we needed to light a fire under the backsides of the teams above us."
Watford: Lee, Chambers, Cox, Demerit, Darlington (Ardley 62), Eagles (McNamee 72), Gunnarsson, Mahon, Jackson (Webber 56), Bouazza, Dyer.
Subs Not Used: Chamberlain, Doyley.
Nottm Forest: Gerrard, Curtis, Morgan, Melville, Doig, Perch, Evans (Thompson 67), Powell, Commons, Dobie, Taylor.
Subs Not Used: Doyle, Bopp, Gardner, Harris.
Goals: Commons 45, 79.
Ref: P Melin (Surrey).