Why is it I know it and They know it but He knows it? But then I knew it, They knew it and He knew it?
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In many languages, and in Indo-European languages in particular, verbs are conjugated according to person and number. In other words, the ending of the verb changes based on the person of the subject (I, you or he/she).
In the past, all Indo-Europan languages used to have verb conjugation, including English (see Old English verb conjugation---the ending in the 3rd person contained -þ/ð, the modern day "th", which is similar in pronunciation to s/z). But as time passed and languages changed, some of them have lost verb conjugation and now indicate the person by prepending a pronoun (I, you or he/she). One example is Norwegian, where even the copula ("to be") lost conjugation. In English this process was not complete. The -s in the third person is a remnant, and the verb "to be" has a different form in all persons.
You might find it interesting that some languages conjugate verbs not only according to the person of the subject, but also the person of the object. In Hungarian (which is not Indo-European), "I know him" is "ismerem" while "I know you" is "ismerlek" (this is different from "tudok" which means "to know a fact").
You want things to make sense? English is not the language for you, then. All past tense are conjugated the same, regardless of 1st person, 2nd person, etc.
I sang, he sang, they sang. We wrote, you wrote, she wrote...
I'm sorry, we have a strange language. The answer to most of these questions is "historical reasons".