I know the following sentences are wrong. I just don't know why. Could somebody explain why they are wrong?

"This floor needs vacuumed."
or
"The baby needs changed."

It should be: This floor needs vacuumming.
or
This floor needs to be vacuumed.

I understand your frustration!

As you know, "The walls need to be painted" is correct, so why is "The walls need painting" correct
but "The walls need painted" incorrect?

This is a very interesting question which I've thought about over the years but never found an answer written in any book.

I have, however, found some interesting examples where the construction (subject) + (need*) + (*-ing) is actually incorrect.

Study the following examples for insight into this matter.

a. Someone should paint the wall.
The wall needs painting.
Correct.

b. Someone should wash the dog.
The dog needs washing.
Correct

c. Someone should pick up that can.
The can needs picking up.
Strange/incorrect

"a" and "b" are typical native utterances, but "c" sounds odd, even wrong. But why? Think about it a moment before reading on.

It seems to me that a gerund conjures up the action in the middle; an ongoing activity taking place stretched out over a period of time.

We think about painting a wall and washing a dog in this way, but picking up an old soda can is just too short and abrupt an action to feel appropriate for this construction.

In the future, should you have any doubts, you could test whether an idea would lend itself naturally to this construction by trying some quantifiers, such as "some" or "a lot of", before the gerund to see how much sense it makes.

Can you imagine "a bit of painting, here and there"? Can you imagine "about two minutes of vacumming"?
I suspect you can.

Can you say the same for "picking up an old soda can"?
I suspect that you can not, because you either pick it up or you leave it there - there couldn't be any question of how much picking up you should put into the job.

By the way, I have heard rural dialects here in the US which do use the construction exemplified by "the floor needs vaccumed." I wonder if anyone else has ever heard a native use this construction.

The wall needs painting.

In this sentence, the word 'painting' is a noun. More specifically, it's a gerund: a noun formed from a verb.

The wall needs to be painted.

In this sentence, 'to be painted' is an infinitive phrase which plays the role of a noun. An infinitiv phrase can play the role of a noun, an adjective or an adverb.

Therefore, there is a pattern emerging with the construction of these sentences. The object of 'need' is a noun every time.

The wall needs painted.

In this sentence, the word 'painted' can only be an adjective, which is why it is incorrect. Of note, the word 'painting' can also be an adjective which is possibly a link to why some use 'painted'. I mean, what's a present active participle and a past passive participle between friends?

The wall needs painting.

In this sentence, the word 'painting' is a noun. More specifically, it's a gerund: a noun formed from a verb.

The wall needs to be painted.

In this sentence, 'to be painted' is an infinitive phrase which plays the role of a noun. An infinitiv phrase can play the role of a noun, an adjective or an adverb.

Therefore, there is a pattern emerging with the construction of these sentences. The object of 'need' is a noun every time.

The wall needs painted.

In this sentence, the word 'painted' can only be an adjective, which is why it is incorrect. Of note, the word 'painting' can also be an adjective which is possibly a link to why some use 'painted'. I mean, what's a present active participle and a past passive participle between friends?

GM is correct, but also remember that, to use the construction "(sub) + need* + gerund"

Verbal must be long enough to be thought of as a gerund, such as:

The wall needs to be painted. (good)
The wall needs painting. (good)

Quick action
The can needs to be picked up. (good)
The can needs picking up. (strange/bad)
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And must be an activity:
The stop sign needs to be seen. (good)
The stop sign needs seeing. (bad)

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