① She did not know which way to go.

= She did not know which way she should go.

② I don’t know what to do.

= I don’t know what I should do.

③ Tell me how to drive.

= Tell me how I should drive.

④ Let me know when and where to meet next time.

⑤ We are fascinated by the mysterious migration of the birds. What tells them when to leave and where to go?

⑥ Birds have wings with which to fly away from danger.

= Birds have wings with which they can fly away from danger.

⑦ This is the only acceptable circumstance under which to break the contract.