Corrected sentence (clear and natural): The pelican caught exactly eight fish.
Why the original sounds wrong:
- The original: "Eight fish is exactly how many the pelican caught." — sounds awkward because of subject–verb agreement and word order.
- "Eight fish" is a plural noun phrase, so if you keep that phrase as the subject you would normally use a plural verb: "Eight fish are..." — but that still sounds clumsy in this construction.
Better options and why they work:
- Active and natural: "The pelican caught exactly eight fish." — clear, simple, and correct.
- Passive: "Exactly eight fish were caught by the pelican." — also correct, but more formal.
- Keeping the original idea but fixing grammar: "Eight fish are exactly how many the pelican caught." — grammatically possible, but still awkward.
- Using the word "number": "Eight fish is the exact number the pelican caught." — this works because "the exact number" is a singular idea, so "is" is correct.
Short rule to remember: Use a plural verb with a plural subject ("eight fish are...") — but often it's clearer to rearrange the sentence: say who did the action first (the pelican) and then how many it caught.
If you want, I can give a few more example sentences you could use or practise changing similar sentences.