It's hard to find somebody who knows cosmology.

It's hard to find people who know cosmology.

It's hard to find a person who knows cosmology.

How do they differ from each other? And which one should I say?


They are all fine, all synonymous; use any of them. If I were composing, I would probably use the last one.
Ah alright. About the way I wrote my question, should I have said 'And which one should I use' instead? How would you ask this sort of question?

Thank you

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You can use either "Which one should I say?" or "Which one should I use?" Say does seem to limit it to speech, but all the time we use the word "say" to refer to things in print.
I see. GG, when you say 'all the time', do you mean always or most of the time?

Thank you

Ah, interesting. I used it idiomatically and didn't realize it.

"All the time" actually means "with great frequency" and not "always."
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Okay. Thank you.Emotion: smile