hypothetically speaking, if you were to cast a love spell on someone who is in a relationship, you have a strong desire and a strong spell, you cast it at the right time and do everything correctly, it could go either way. once you cast the spell it's in the universes hands. the person could feel the spell on them, but subconsciously will be fighting back because we all have free will. each persons free will however is different. if his is weak, then the spell will work quicker and last longer. if it's a strong free will, the opposite will happen. either way, if the spell does take effect and he leaves the person for you, it's a ticking clock, he could break free of the spell in an hour, a day, a year, who knows, therefore you would need to recast often, and really, that's not love.
if, on the other hand, the spell backfires, the person might end up hating you [common outcome] or nothing will change between the two of you, yet you begin to notice you're very unlucky in love. this is why we think long and hard before we cast, because even the most skilled witch in the world can cast a spell for the wrong reasons and have it backfire.
as Lark said above though, learn the basics before you think about casting spells, it will help you understand magick and prevent you from casting unwisely. [but we're all human, slip up and curiosity do get the better of us from time to time.]