Despite slow starts to both halves of the second qualifying round clash, Motherwell dominated against a side sitting top of the Icelandic league following 11 matches.
Forbes missed one of several decent first half opportunities created by the hosts, who survived two scares in the opening moments of the second period.
But the midfielder, who netted three goals in six European games last season, found the net from 20 yards in the 62nd minute.
The victory on a lush new Fir Park surface, which coped well with a second consecutive evening of torrential rain, allayed fears that Motherwell's lack of competitive action would leave them susceptible to an upset.
The visitors struggled at the back at times, although their young forward line showed enough glimpses of ingenuity to suggest they will pose a threat in Iceland next Thursday.
Motherwell handed former Charlton goalkeeper Darren Randolph a debut but Craig Brown's starting line-up otherwise had a familiar look with his bench filled with youngsters as he continues to endure a frustrating summer in the transfer market.
However, Breidablik also had youth at their core with 20-year-old Kristinn Steindorsson and Alfred Finnbogason, a full Iceland international at 21, attracting interest from top-flight clubs across Europe.
The pair, playing just behind and wide of a lone striker, saw much of the ball in the opening stages but Motherwell created two good chances just before the 20 minute mark.
The first came when Keith Lasley flicked Stevie Hammell's cross into the path of John Sutton but Elfar Helgason scrambled clear.
Jamie Murphy, playing up front in a 4-1-4-1 formation, soon saw his flick from Forbes' cross held by Ingvar Por Kale. Finnbogason unlocked the Motherwell defence with a first-time pass but Hammell forced Gudmundur Petursson to slice wide.
The visitors were looking vulnerable in defence though and Tom Hateley beat two men before Forbes headed his deflected cross over from 10 yards.
Dangerous crosses from Chris Humphrey and Hateley just evaded Sutton, and Murphy got in behind before Kale blocked from a tight angle.
The goalkeeper unconvincingly scrambled clear Sutton's cross in the final seconds as Lasley and Humphrey ran in, but Motherwell made a sloppy start to the second half as the visitors squandered two glorious chances inside three minutes.
Forbes gave away a needless free-kick 35 yards out and the home defence allowed Gudmundur Kristjansson to spin undetected at the far post before his cushioned volley across the face of goal drifted just beyond two team-mates.
The unmarked Steindorsson then sliced a volley wide from six yards after Kari Arsaelsson's free header.
Motherwell settled and Forbes placed a 30-yard free-kick just wide before Kale flapped the ball behind after being caught out by Hateley's swerving set-piece.
Debutant Gary Smith replaced Humphrey with the 18-year-old playing up front alongside Sutton and Murphy moving wide in a 4-4-2.
The breakthrough soon came after Hammell returned the ball to Forbes following the midfielder's half-cleared right wing corner.
The 21-year-old immediately shaped inside before his curling effort just evaded Smith but nestled inside the far post.
Motherwell continued to press without creating chances but Brown's side sat deeper in the closing 15 minutes as the visitors pushed men forward.
However, Randolph comfortably held Kristin Jonsson's 30-yard free-kick and sub Steven Saunders nearly added a late second when his header was deflected just wide following Hateley's corner.