He did not speak clearly and/or correctly.

[sitifan]

1. He did not speak clearly and correctly.
2. He did not speak clearly or correctly.
Which of the above sentences do native speakers think is correct?

 

[emsr2d2]

Rather than asking us which we "think is correct", you should ask us which one we use [most often]. If we use both, there might be a difference in context.
What prompted you to ask the question?

 

[sitifan]

Rather than asking us which we "think is correct", you should ask us which one we use [most often]. If we use both, there might be a difference in context.
What prompted you to ask the question?

https://kknews.cc/education/65vybkv.html
According to the above link, the Chinese teacher of English says:
"He did not speak clearly and correctly. = He spoke clearly but not correctly.
He did not speak clearly or correctly. = He spoke neither clearly nor correctly."
Do native speakers agree with what he/she teaches?

 

[5jj]

No.

There is no significant difference in meaning between the two.

The first groups together 'clearly and correctly'; he did not speak in this way.
The second treats the adverbs separately; he spoke neither clearly nor correctly.

 

[sitifan]

https://kknews.cc/education/65vybkv.html
This film is not interesting and instructive. = This film is interesting but it's not instructive.
She cannot sing and dance. = She can sing but she cannot dance.

Do native speakers agree with what he/she teaches?

 

[5jj]

No.

 

[Tdol]

Do native speakers agree with what he/she teaches?

Not at all.

I'd say the distinction, if any, is this:

He did not speak (clearly and correctly).
He did not speak (clearly) or (correctly).

It's certainly not what this person says it is.

 

[Tarheel]

Well, you have already gotten to a second question. (That was fast.) If somebody says of someone, "She can't sing and dance" it tells me she can't do both at the same time. In fact, there is more that it doesn't say than what it does say. For example, it doesn't say if she can either sing or dance. (Maybe she can do both but just not at the same time.)

As for your original question, if a person doesn't speak clearly you can't tell if they spoke correctly or not. That's because you didn't understand what they said. So it doesn't make sense to say a person didn't speak clearly or correctly. You can only be sure of one or the other.