I would have never done that. Someone told me that ONLY "I would never have done that" is correct. I always heard both and read both. If it is not correct could you please tell me why?
It's not so much that it's not "correct". Correct English is a myth that English teachers spread.
It's unusual, though, because the usual rule for negative placement requires that the negative go after the first auxiliary verb. The first auxiliary verb in this sentence is would, so it's normal to say
- I would never have done that.
However, although would /wʊd/ is the first auxiliary verb, it's contracted with the following auxiliary verb have /hæv/ in speech, to form a complex auxiliary would've /'wʊdə(v)/; the final /v/ is usually dropped, especially before a consonant.
Some people may interpret would've as a single auxiliary verb -- especially since it works as a semantic union, referring to a judgement of past likelihood. If so, then they can use it as the First Auxiliary Verb within the meaning of the usual rule, and put a negative after that.
- I would've never done that. (/'wʊdə'nɛvər/)
this would always be contracted in speech,
but in spelling, many people avoid contractions,
even though they would read it aloud that way.
- I would have never done that. (pronounced as above)
Some can even recontract that negative; I've heard native speakers say
- I would'ven't done that. /(/'wʊdəvən'dənðæt/)
instead of the more normal
- I wouldn't've done that. /(/'wʊdənə'dənðæt/)
So, it's not wrong, it's just a different interpretation.
Let this be a lesson about the value of others' judgements on "correctness".