Hello it's the first time I'm posting +__+
Anyway, i came across this phrase when i was going through some grammar questions.
"speak bad of".

The sentence was
"You ought not to speak bad of others in their absence."

Now, my grammar book tells me that this phrase is wrong,
and says also that "speak _____ of "
can only be used with "well/ill".
(so the correct sentence would look like, "You ought not to speak ill of others in their absence.")

However, I have come across many articles/writings/dialogues/etc....
where native speakers would use the "speak _____ of " phrase with many other words, such as [nicely, good, bad, highly......]
I don't believe there is a specific limit to what words you can put in that blank.(But then what would I know, I'm not a native speaker. ;p) I have googled the term and it turns up in some articles, video titles, and casual diary entries also.

In conclusion, is using "bad" instead of "ill" grammatically incorrect? Is this term something that is fixed or can be modified to fit the speaker's needs?
I'd like to know. Thanks!
Both "ill" and "bad" are adjectives. We need an adverb to modify a verb, not adjective.

He speaks extremely well.

He ran off quickly trying to catch the bus.

He speaks highly of you.