Today I've been doing some tasks and I've found one interesting thing as for me. So I'll explain what I mean.

Look at these sentences:

My friend is 19 years old.
So is mine.

where mine refers to my friend (another person).

And now I want you to look at this sentence:

My friends are 19 years old.

Should the response be "So are mine" or "So are mines"?

I know there is a verb "to mine"(like to dig) I know but I'm really confused with it.

If I understand it well, "Mine"(NOT A VERB) works as for singular (A friend) and plural (friends).

closed as off-topic by FumbleFingers, AndyT, Cascabel, Scott, J. Taylor Jul 21 '18 at 9:31

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  • Mine, yours, his etc. are "pronouns", for which the corresponding "possessive adjectives" are my, your, his, etc. Note that his has the same form in both usages, so whereas it's So is mine = So is my friend and So is yours = So is your friend, the third person singular versions are So is his and So is his friend. – FumbleFingers Jul 16 '18 at 17:34
  • @Max, does it seem helpful, or not, that you chose to see things that way, please? If you’re seriously asking: ‘Should the response be "So are mine" or "So are mines”?’ Then no; never either. – Robbie Goodwin Jul 16 '18 at 18:20

The correct form is as you predict:

My friends are all 19 years old.

So are mine.

"Mine" is a possessive pronoun — it functions as the subject of the sentence, taking place of (in this case) "my friends." It does not change form based on any grammatical properties of the antecedent, including number, and so it takes the form "mine" regardless of whether the original subject was singular or plural. However, the verb which follows it will change based on the original subject:

My friend is 19 years old.

So is mine.

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