My girlfriend has long hair, but I want her to look mod, gorgeous and hot. Which haircut should I suggest her? I am sure she will approve what I say, as she loves me.

That's nice, I mean if your girlfriend really loves you, she should.

About hair, I want to first tell you, that neither all ladies look good in haircuts, nor long hair look bad on all. It depends upon person to person, but yes - styled hair - for which the use of scissors can play a critical role, surely look good, nice, lovely, beauteous and attractive, rather tempting. You should suggest a cut, by looking at her face, its structure, her age, her colour-complexion, her job, her nature, and many other things. For example, for a chubby, round face, with fair complexion, the pixie cut, or a cut in which hair hang around, like bob cut will work wonders. She will look cuter. If her face is longer, shoulder cut will do, but - what if she prefers long hair... in that case, ask her to get her hair straightened and trimmed, from ends.

Take some cues from here -
http://www.ukhairdressers.com/hair_move/

Tell me her facial structure, and I shall try to help, if I can.

You want. She loves you, not one peep about you loving her. You are sure she will do this for you.
Good grief. Gorgeous has nothing to do with how long or short a woman's hair is. Work on your own looks. She's not a mannequin. For all we know she might look better with long hair.

What style hair does she like ? Long hair is sexy to many people, why should she cut it, unless she wants to cut it ?

Perhaps she can go to a professional fashion expert to give opinion from the shape of her face, her complextion and more, what hair or make up would be best.

It has been suggested privately that I was snarky and perhaps I could edit my response. I read it again, and I feel the same way, and don't mind being snarky in this case.

The last time a boyfriend asked me why I didn't 'do' something with my (long straight) hair was about 35 years ago, but I still remember vividly saying 'What do you want me to 'do' with it, take it out to dinner?'

That boyfriend wasn't usually one to dictate my appearance (we are friends now, all these years later) but I resent men who do. I don't like the rest of the question either.

I agree with joypulv, why try to change the way your girl looks. Let her have her own style. You say she loves you and I would hope you love her. Just realize that wanting her to change her appearance as subtle as it may be it seems to me like you want to control your girlfriend, you may not even be aware that you are doing it but you are at least subconsciously trying to enact some form of control over your girl. So I ask again, please let her pick her own style, if you truly love her it won't matter what hairstyle she has. My answer may seem a bit harsh but it's the honest truth the way I see it.