When you pin your hopes on something your expectations hopes or dreams are dependent on another person or thing. To pin your hopes on something means the outcome is out of your control.

1. We shouldn't be pinning our hopes on Charlie to get the deal over the line. He's not good at negotiating under pressure.
2. I've pinned all my hopes on getting this job, so I'll be devastated if it doesn't come through for me! 
3. Everything else he's tried has failed, so the chance that he can somehow find an investor to save his business is all Kevin has got to pin his hopes on now.

1. I pinned all my hopes on passing this exam and now that I have failed I don't know what to do.
2. I find it difficult to pin all my hopes on one area of my life I much rather have diverse interests to spend my time on.