Had Margaret realised she would be travelling alone, she ____________.

• Perfect conditional/ 3rd conditional:
- ১ম অংশে Had + sub + V3 এবং পরের অংশে sub + would have/could have/might have + V3 + extension.
Example:
1. Had Margaret realised she would be travelling alone, she would never have agreed to go.
2. Had I known you were waiting outside, I would have invited you to come in.

Source: Cambridge Dictionary Grammar.