I couldn't think how to word the title without writing my whole question, I am a native English speaker so while I understand saying "my legs aches" is wrong and sounds weird, I have no idea why. Same as saying "my back ache" is also weird.

  • "Leg" without an -s is the singular while "legs" with an -s is the plural.

  • "Ache" without an -s expects plural agreement while "aches" with an -s expects singular agreement.

Why do the singular (and respectively, plural) forms not agree in terms of '-s' or 'no -s'?

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    The third person singular present indicative usually adds an -s in English. For example he eats while they eat. Similarly you write but she writes. See english.stackexchange.com/questions/405449/… – Henry Aug 15 '18 at 11:00
  • Hello, and welcome to EL&U. I've edited your question to highlight what appears to be its intent. Please feel free to edit further or to roll back the changes. – Lawrence Aug 15 '18 at 11:19
  • This question, as originally formulated, manifested a simple misunderstanding, which was corrected by the two answers below. After the edit, it seems to be asking why is adding and s used for two entirely different purposes in English: forming the plural of a noun, and forming the third-person singular of a verb. Chances are that this new question is unanswerable (although it would be really exciting if it turned out that there is an answer to it). – jsw29 Aug 15 '18 at 16:37
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    Pity. I believe that the question should have been migrated to ELL. – Mari-Lou A Aug 15 '18 at 16:38

There is a slight misconception in your title.

Your legs (plural) ache and your back (singular) aches.

ache and aches are not singular and plural, respectively. aches merely has the third-person-singular -s ending in conjugating verbs in the present tense:

I ache
you ache
he/she/it aches
we ache
you ache
they ache

In your example, legs ache is the plural third person, while back aches is singular third person. It is just one of the many exceptions in English that we just have to remember.

  • Apologies - I edited the OP's question while you were formulating your answer. The question now references singular/plural agreement for the verb. You may wish to update your answer. – Lawrence Aug 15 '18 at 11:18
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    @Lawrence Thanks for the hint, but I think the answer can stand as it is. If you see any additional remarks that improve the answer, feel free to make an edit! – Ian Aug 15 '18 at 11:21

The word ache in "my legs ache" is a verb, it is not a noun.

You can have a backache (back-ache), and a headache, but when a part of your body is hurting you or complains of a dull constant pain, we say that it (singular pronoun) aches.

My shoulders (pl. n) ache from bending hours over my laptop
My back (s. n) aches after painting the ceiling

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