How common are these grammatical errors in English-speaking countries besides the US?

Grammatical mistakes and idioms differ by country and region. So other English-speaking countries have their own quirks, a few of which overlap, and others don’t.

(Also, in general, I’d say the USA is not particularly high on the list of nations with best-educated population nor most-grammatically-correct speakers, also because of the popularization of slang, and the anti-intellectualism.)

“Incorrect formation of the antecedent clause of a conditional sentence: If you would’ve asked him, he would’ve told you.” What? What is wrong with that sentence?
”...a conjunction instead of a relative pronoun…”
Use of the base form of a verb instead of the past participle when using the passive voice or a derivative adjective…”
It all leaves me baffled.
I hated English with a passion. It was the only subject I hated.

Interesting.

#3 I find is very common among ESL first language Spanish, but I don’t think I hear it from people born in America.

Probably #2 is fairly common for Spanish speakers too. Conjunctions are tricky, and even worse propositions are a nightmare. Very difficult for them to master the prepositions. It’s understandable if you speak Spanish.

Zissou while Dutchess had had had had had had had had had had had the more mellifluous tremolo in the opinion of the pedagogue.

In discussions of prescriptivism versus descriptivism, Old English says it best:

Se wisa wer timbrode his hus ofer stan.
Þa com þær micel flod, and þær bleowon windas, and ahruron on þæt hus, and hit ne feoll: soþlice, hit wæs ofer stan getimbrod.

Þa timbrode se dysiga wer his hus ofer sandceosol. Þa rinde hit, and þær com flod, and bleowon windas, and ahruron on þæt hus, and þæt hus feoll; and his hryre wæs

The wise man built his house on stone.
Then a great flood came there, and winds blew there, and fell down upon the house,and it did not fall: truly, it was built on stone.
Then the foolish man built his house on sand [lit sand-gravel]. Then it rained, and a flood came there, and winds blew, and fell down upon the house, and the house fell; and its fall was great.

^Ain’t nobody ‘cep JLeslie gon’ answer the muhrfuggin question? Dis genrul, y’all.

If I understood the question I could answer it but I don’t. I know it’s a lacking on my part. However, I am certain that confused / improper grammar is not limited to American English.

I don’t put “If you would have asked him, he would have done it,” in the same category as “had had had had had had.” I still don’t see what it is wrong with it. What other way is there to say it?

The correct form would be, “If you had asked him, he would have done it.” The “If…” part talks about what did not happen in this case. The part after the comma is where the would belongs, because that’s where you are saying what would have happened, had the condition specified in the “If…” clause occurred.

Using would in both clauses suggests that the condition in the “If…” clause is itself dependent upon some further condition which isn’t mentioned. Or it would suggest that if people thought about it, which they don’t. This error has apparently become so common in the US that people don’t even notice it or know what the correct alternative is.

I’m not saying or insinuating anything about whether US speech is more incorrect than that of other English speakers. I’m asking just what I asked. There are (or used to be) a fair number of language nerds on this site.

Did you send the Q to Jeruba and janbb?

I got ya @Zissou. It’s redundant. Thank you.

I have also heard that when Spanish is taught as a second language, they always teach the formal Spanish, not the every day Spanish that people actually speak. The every day Spanish, and English and every other language, is always going to contain inconsistencies and errors as it progresses.

^^The formal verb tense is used in Spanish. We use it when addressing older people, or people who are not “familiar” similar to how we use Mr. or Ms. in English. In fact, I would say the formal is used very regularly in Spanish. When I don’t know how to conjugate a verb in Spanish to the formal and I am speaking to someone older than I am, or if I simply forget to conjugate correctly, and realize my mistake, I feel badly, or sometimes even apologize for my language skills.

The second person plural is not used much, especially not in Latin America, but it’s understood. Second person plural is informal, but most people when addresses a group use ustedes, which is technically formal, but used for all groups by most counties in Latin America.

If they’re common, are they really errors?

Yes.