Here are three sentences to help me ask this question:
- The cats are all that they own.
- All that they own is the cats.
- All that they own are the cats.
The first sentence is definitely correct as written. However, I'm not sure whether 2 or 3 is correct.
To explain my doubt, take another couple of examples:
- The clothes on their backs is all that they own.
- The clothes on their backs are all that they own.
To my ear, both of these sentences sound correct, but their meanings are different.
The first sentence is saying, "The only clothes that they own are the ones on their backs."
The second sentence is saying, "The only thing[s?] they own is[are?] the clothes on their backs."
The fact that it's difficult for me to word this second sentence in the inverted format is why I think I'm having trouble.