In this use, "key" is an adjective, not a noun. Therefore, there is no plural.
Interested in Language
I hope you would be able to help me with the following sentence, please - which of the following versions is the right one, and why:
1. Listening, caring and understanding are all key to a successful relationship or
2. Listening, caring and understanding are all keys to a successful relationship
It seems like there should be an "s", meaning "keys" (because we're dealing with multiple actions), but it doesn't ring right.
Furthermore, when searching on Google for the exact combination of "are all keys to a successful", I found ~448K appearances with of "keys" (plural), but more than 2 million (more than quadruple) the same exact sentence with "key", meaning singular.
Would appreciate your help!
Both are equally correct. It depends on whether you consider "key" to be a noun or an adjective here. In my opinion, "key" is the more common usage in AmE today, but there is nothing wrong with "keys." Google is not a reliable way of researching questions about English usage. You should consult instead COCA, the Corpus of Contemporary American English.
Thank you very much, that's very helpful!
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Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.