FWIW: there are two rules at work here: (1) for regular verbs, only the third person singular takes the -s form (e.g. he knows); (2) the verb number must agree with the subject number. Since the subject here ("ones") is plural, we use third person plural: " [they] know". – Chappo Hasn't Forgotten. Dec 1, 2018 at 23:23. More @Wikipedia
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