agreement - Why do my legs (plural) ache (singular) but my back (singular) aches (plural)

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'?

Answer

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.