Ai is like a yume, while yume-tachi is like ai. In a yume you imange anything, whether it's true ai in real life or ai that you wish to have. Yume-tachi can have endless possiblities, you can be in a feild of sakura-tachi (cherry blossoms) blooming, sitting under a cherry tree with someone you ai. Or you can be fighting a battle to(english) protect someone you care about or to (english) get revenge from someone that dishonor you or the ones you hold close to you. Theses are what yume-tachi can be for me or anyone else who holds such strong emotions.