Did you know about this before?

No, I usually just decide whether to use commas or not based on vibes honestly. Good to know though.

yes, learned about it in middle school english class

Today I learned.

I'll be real I have no clue what the fuck apposition means and I have a feeling it's a stupidly complex sounding word for a simple concept

If someone says apposition one more time I’m gonna freak out

I've always intuitively applied the rule but never actually thought about it or knew what it was called, interesting

To give a brief summary for those who don't know what this means: If the added information is necessary to clarify something, don't use a comma. In their example, this speaker has more than one friend, so to clarify which friend, they add the name Alice. (It could also be that the speaker knows more than one Alice, but only considers one of them their friend). If the added information isn't necessary to understand the original intent, you should use a comma to separate it from the "important" information in the sentence. In their example, the speaker can only have one eldest sister, so the name of the eldest sister isn't important; thus, the whole sentence could have just been, "My eldest sister is 35."

Yes, I was taught this in Grade 2. Which is also when we were taught when to capitalise words like Mum and Dad. The word appositive and its derivatives were not used, however I have heard this word a lot from primary school, literacy support, and EALD teachers in recent years.

I do not have a grammar book in front of me, but that seems backwards. "My friend Alice" would mean one of my friends, the one named Alice (not Bill, Cate, etc.). "My eldest sister, Hannah" would mean that Hannah is the name of my eldest sister (a unique position, as I can have only one eldest sister). The condition of being my friend is not restricted to Alice, while the state of being my eldest sister is restricted to Hannah.

I want to know more about your username lol

It might be easier to think about whether or not the name is necessary to identify who you're talking about. Describing someone as your eldest sister is enough to identify her, because that can only apply to one person, so her name is optional and in between commas. The friend example assumes you have more than one friend--in that case, their name is needed to identify which friend you're talking about.

If one person exists in the group then use commas, but you otherwise do not use them. For example:”His wife, Sarah, works weekends,” but “His friend Hannah works weekends.”

I know this as a native speaker, but only because my Arabic teacher used it to explain the Arabic version of this (Badal) to my class. Otherwise, I'd have no idea what this is.

I had to google this to understand. The explanation in the OP doesn't really make it clear. Basically "apposition" means you're referring to the same noun twice. So "alice" and "my friend" both refer to the same person/place/object, so they're in apposition. Then I had to understand restrictive and non restrictive. Non restrictive uses commas, and it's any time the apposition isn't necessary. So in the example, "my eldest sister" is clear already, you cant have multiple eldest sisters. So Hannah isn't restricting the phrase at all, it's already limited to that one person. Since its non restrictive, you use a comma. But if they had said "my eldest sister" and that person somehow had two eldest sisters (twins born at precisely the same time?), then it would be restrictive, since the name Hannah is restricting it to just one of the eldest sisters. So then you would remove the comma. Another example: My dog Odie loves lasagna. (Write it this way if you have multiple dogs, since you're restricting the "my dog" phrase to just Odie) My dog, Odie, loves lasagna. (Write it this way if you only have one dog, since its non restrictive since you're not narrowing down/restricting the phrase "my dog.")

I thought the commas in Hannah were because you were giving the extra information that it's Hannah, meanwhile on the first one you're not trying to give the extra information that your best friend is Alice, but rather you're just saying it as a whole thing

Well, I knew how that worked but I had no idea what the word "apposition" meant. As a decently educated native speaker, I was completely unable to parse the sentence "As a nonrestrictive appositive, it takes commas." What in the bloody hell is a nonrestrictive appositive? I'm all in favor of learning the grammar, but there comes a point where you'll do more good by just going outside and talking to people

No. This is just encoded as general logic in my head and I don’t know these things about the language. It’s part of why I contribute, to learn about English.

This is something they teach us in grade school, but I didn’t know there was a word for it.

I remember learning those comma rules as a child, but I didn’t know the name for them!

I'm a native speaker, studied English and was top of my class (albeit a couple decades ago!). I have never heard of appositions, restrictive or not. Honestly I think studying grammar when learning a language is almost pointless. If you watch, listen, and read enough then you learn things like this intuitively. 

In legal writing we often use appositions. John, hereafter Defendant, signed a contract with Steve, hereafter Plaintiff, on June 9. I call them asides. It's like a short definition, a reminder of something mentioned earlier, or an attempt to quickly clarify something. Can also be done with parentheses. The house (*715 Main Street*) had a green lawn. When using parentheses the words are often italicized to read easier.

In legal writing we often use appositions. John, hereafter Defendant, signed a contract with Steve, hereafter Plaintiff, on June 9. I call them asides. It's like a short definition, a reminder of something mentioned earlier, or an attempt to quickly clarify something. Can also be done with parentheses. The house (*715 Main Street*) had a green lawn. When using parentheses the words are often italicized to read easier.

I didn't know the rule, but I did know it intuitively

Never seen that word in my life

Yeah didn't know the term for it but I did know about that kind of sentence where you should be able to take the part in commas out and it still makes sense.

Yes. Extra information you don’t need about the subject is set off with commas. Information that defines the subject isn’t. My friend John, a chef, enjoys long walks in the park. John the chef, not that other John the dog groomer, enjoys long walks in the park.

A lot of what native speakers do with punctuation is just unconscious habit, learned from reading. (Or *not* learned, from *never* reading, as is often the case.)

I was advised that for names, if it makes sense without the commas then you don’t need them. Names are typically understood as names, and it isn’t always necessary to use a comma. The reason “Hannah” uses a comma in this example is because you are explaining something about her. Typically when using these descriptors, the sentence makes sense when removing the descriptor (Hannah). I don’t know if that has a fancy grammar term. ;) My eldest sister, Hannah, is 35. My eldest sister is 35. (Which sister? The eldest.) My sister Suzie likes jelly beans. My sister likes jelly beans. (Which sister? We don’t know.) Joe is dancing. (No comma!) My friend Alice is silly. (No comma, because it’s clear you mean a person named Alice!)

Yes, but it’s a detail you only learn and remember if you’re really into grammar. 

I didn't remember the term apposition, but I knew there was a rule about whether or not to set something like that between commas. The word appositive seems more familiar to me. Based on what I remember from high school: As someone else said, it has to do with how many (sisters, friends, etc.) there are. In this example, you can only have one "*eldest* sister," so that sister's name is like an aside—extraneous information you're supplying—and gets separated by commas. But, assuming you potentially have several friends, adding the friend's name is necessary to clarify which friend you are speaking about, so it's like part of the noun phrase, or whatever you want to call it. If you said "my friend with the blue hair," it would be the same idea. You wouldn't offset "with the blue hair" with commas. I suppose it still applies if the listener/reader would have no idea how many siblings, pets, etc., you have, so if you say "my sister, Hannah," (with commas), that suggests you only have the one sister, but if you say "my sister Hannah" (no commas), the implication is that you have more than one sister.

Yes, it is about explanatory apposition vs. specifying apposition (that's what they are called in Romance languages, at least in the ones I know)

This is my friend, Alice. My friend Alice likes puppies.

No; it's bullshit. Just another bad teaching example. "Apposition" is a word, but you'll never hear it. They probably meant to say "opposition" - but it's still crap.

I did know, I learned it in high school.

I don't know precisely why it works this way, but I know that it does. I've never put any conscious thoughts into it, but my brain automatically knows whether to put the comma in.

Yes

I have no idea what the word apposition is

I know to use commas for "my eldest sister, Hannah, is 35" and not for "my friend Alice" but it's not something I was ever taught explicitly. I don't know for sure if I've ever seen the word apposition before now.

what the what?

I can honestly say I've never heard of this rule in my life

Yes, probably mostly because we have the same thing in French.