Before the "and", "but", "so", "while", etc. — while most European languages, indeed, (usually) require commas before conjunctions, English is following a somewhat reversed convention. Well, at least the author remembered that they need to omit a comma before "that".
i dont think anyone actually cares unless you are in english class or someone wants to feel smug on the internet (you can ignore the 2nd one completely).
Informally, commas are "misused" regularly even by native writers, and almost no one notices or cares unless the misuse is frequent and egregious.
Formally, most comma use is an issue of style, not grammar, and style guides differ on when they should and shouldn't be used. The main rule that's actually a matter of grammar is this: commas cannot be used to join two "clauses" (phrases which could be independent sentences) without a conjunction.
English has no central authority, so the lines between grammar and style can get fuzzy. Most guides would say you need an "and" after the second comma, but some make an exception if the clauses are "very short and alike in form". See Wikipedia's article Comma splice if you are interested in the pedantics. https://en.wikipedia.org/wiki/Comma_splice
You still didn't answer: am I supposed to write "I'm eating a hot dog she's listening to radio" without a comma for that sentence to be grammatical? Or am I supposed to always insert "and" into it?
With or without a comma, without a conjunction like "and" the example is a run on sentence and is ungrammatical. You should join the clauses with a conjunction, and including a comma before the conjunction is a common style.
There's a caveat where some style guides permit using a comma without a conjunction in specific contexts, but most don't.
So it's "You do need a comma before a conjunction, if it connects two independent clauses" then — "you need a comma before a conjunction" and "it connects two independent clauses" are two fully formed simple sentences, each having its own subject and predicate. See what I mean?
And almost nobody writes sentences like "I love her and she loves me back" with a comma before "and".
A subordinating conjunction like if turns an independent clause into a dependent clause, so you don’t use a comma.
> And almost nobody writes sentences like "I love her and she loves me back" with a comma before "and".
In informal writing, people frequently leave out commas for everything but lists.
But it’s also very common to see it used “correctly”. In any kind of writing where someone would be expected to proofread what they wrote, I’d expect it.
The comma prevents readers initially grouping words incorrectly.
“I called John and Mary answered.”
Your brain will start to parse that as “I called John and Mary” before backtracking once you hit the last word.
