An important rule in English grammar is "subject-verb agreement". It basically states that a verb must conform to the form of the noun (i.e. Singular/Plural). My question is:

What is the purpose of this rule?

It doesn't seem to play a huge role in semantics at all. "He drinks coffee" and "He drink coffee" both make sense and the -s at the end doesn't seem to add to the meaning of the sentence at all

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    It's generally pretty futile to ask for "purpose", as if languages were designed. This particular usage is the historical remnant of a time when there were distinct English verbforms for sg1, sg2, sg3 and pl; most of these slipped away gradually. – StoneyB on hiatus Dec 23 '16 at 16:16
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    "The sheep drink(s) coffee." – Greg Lee Dec 23 '16 at 16:18
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    as others have noted, grammatical forms do not have a "purpose" in the sense of a teleology. but they do have a function, and it's reasonable to ask about the function (purpose) of 's' in "he drinks coffee". it seems we could get along fine without it, but we have retained it. why? – mobileink Dec 28 '16 at 22:04
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    iow, just because something is convention does not mean it is arbitrary or capricious. it became a convention for a reason - it served some purpose. think evolution. it's blind - no "purposes" there - but that does not mean there is no reason. quite the opposite, features survive for a reason, they satisfy some need. – mobileink Dec 28 '16 at 22:08
  • Not the reason though but an interesting aftereffect in Sanskrit is that you can arrange the words in any order in a sentence and iti wil still mean the same. 'We love XMas' 'Love XMas We' "XMas we Love' – ARi Jan 5 '17 at 16:38

Grammatical facts aren't volitional and purposive, they are conventional: you say "He runs", "I run" because that's how your colleagues and your ancestors talk(ed). If you go far enough back in time you would also say "you runst" and "we runneth". You can be understood if you say "He run", and some people do that and we don't even notice. Eventually we might all end up saying "he run".

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    That makes sense. Now that I think about it, assuming that grammatical rules need to have a purpose is strange because we didn't intentionally design them to convey perfect meaning(Credits to you and StoneyB for making me realize this) – user3436836 Dec 23 '16 at 16:29
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    Thou run'st, he runneth, and we/ye/they runnen (if you go back say 600 years) - otherwise +1 – StoneyB on hiatus Dec 23 '16 at 17:29

That is for historical reasons, and it dates back millenia to those times when the ancient Indo-European languages were pro-drop, that is they tended not to use personal pronouns as subjects, many are still like that, for example Polish.

Some examples from Latin: te amo means "I love you" (lit. "you I-love"), but te amamus means "we love you" (lit. "you we-love"). As you can see, there are no subject pronouns in those sentences, still the verb form clearly shows what is meant.

All the modern Indo-European languages, including English, stem from a pro-drop ancient ancestor language, the use of personal pronouns as subjects is a later innovation, and some modern IE languages lost the subject-verb agreement completely, like Swedish or Afrikaans.

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