1) It's standard English. 'You belong with me' is the same idea as 'we belong together.' 'Belong to' refers to possessions. The cat belongs to her. It's pretty weird to talk about a person belonging to someone else for obvious reasons. 2) I didn't know what it meant, it's an American expression but from oxforddictionary: Bleacher: A cheap bench seat at a sports ground, typically in an outdoor uncovered stand: 3) 'I've' is the word that's been left out. I've been here all along... Well from the context Taylor is talking about a boy she likes who doesn't reciprocate her interest. Why can't he see that she has always been there for him? (She's been there all along.)

1) "Belong with" in this song means the girl feels she would make a good girlfriend for the boy she's interested in because she feels they're compatible. "Belong to" suggests ownership as in owning an object or land so it wouldn't make sense for this situation. 2) "I'm on the bleachers" is a metaphor meaning left out of the situation. Bleachers are seats in a stadium that very are high and far away from the activity on the field. 3) The implied words left out of the phrase "been here all along" is "I have" so the complete phrase is "I have been here all along." You could also contract the words "I" and "have" to "I've".

2. Being on the bleachers is like being on the sidelines and watching the action.