Be head over heels
Be in love with somebody very much
- He is head over heels in love with that girl.
Fall in love with
Start feeling love towards somebody
- Slowly but surely she started to fall in love with him.
Hook, line, and sinker
To be completely deceived
- She told him she needed the money for her baby and he fell for it hook, line, and sinker.
Break up/Split up
End the relationship
- She's just broken up with her boyfriend.
Have/Got the hots for somebody
Finding somebody extremely attractive
- She's got the hots for this guy in her office.
Kiss and make up
Make peace after an argument
- Let's kiss and make up.
Somebody who cheats on his/her partner
- The womanising of royal love rat James Hewitt.
Tie the knot
- So when are you two going to tie the knot?
(be sb's) Main squeeze
Committed romantic partner
- My main squeeze cleaned me out last night.