This is the short written form of have, stuck onto the word before it: I've, you've, we've, they've. Most of the time it is the helping have that builds the present perfect — I've finished, they've gone. In speech almost everybody says it this way, even careful speakers; the full have would sound oddly slow. In writing you use 've in messages, dialogue, and friendly prose. School essays and formal letters often still want I have. Though actually, I've can also be the have of owning something (I've no idea, I've a meeting at three) — that owning use is more British, and a bit old-fashioned in American English, where people prefer I have or I've got. You cannot start a question with 've: we write Have you…?, not 've you…? After a name or a noun you may see it (The kids've left), though some teachers dislike that in writing.