Engaged!
Most couples only need you to say "congratulations" when they get engaged. If you want to go further and put it in verse, there are some websites and other sources out there that contain poems that might be right for the occasion.
Instructions
1. Search some websites that focus on poems of romance. Some are listed in the references section. For more, search using keywords such as "engagement sayings." Try other keywords related to congratulations cards or even engagement announcements.
2. Do some research at Hallmark or another gift card store. As cheesy as it may sound, the card verses are written by professionals trained to find those "right" words. You could find just the right poem for the couple in mind. If that fails, spend some time in the poetry section of a book store or library.
3. Check out some lines from the movies. A quote from a movie may seem corny, but it can be extremely personal in an engagement congratulation letter or card. For example, "Sleepless in Seattle" has the famous love quote:
"It was a million tiny little things that, when you added them all up, they meant we were suppose to be together... and I knew it. I knew it the very first time I touched her. It was like coming home... only to no home I'd ever known... I was just taking her hand to help her out of a car and I knew. It was like... magic."
If that's too long, there's always "You complete me" from "Jerry Maguire."