How can I explain to people that the phrase "off of" is grammatically incorrect?
You can't, because it's not.
There are thousands of examples of “off of” in the Corpus of Contemporary American English , not just in spoken English, but in magazines, newspapers, and academic journals as well.
“Off of” is well-established as standard in American English.
Plain “off” may be stylistically preferable in many cases, but it is simply not a rule of English grammar that if a word could be removed it must be removed.
Some people seem to think that such a rule exists.
It does not.
english.stackexchange.com/users/39/nohat
The phrase "off of" has a long history, which, according to the OED, stretches back roughly 500 years.
If you want to consider it to be "wrong", then it certainly has a lot of staying power.
Here are their quotations using "off of": ?
c1450 in G. Müller Aus mittelengl.
Medizintexten (1929)
116 Take a sponfull of e licour..of of e fyir and sette it in good place tyl at it be ny colde, soo as ou mayst suffryn to holdyn
er-in in hand.
a1616 SHAKESPEARE Henry VI, Pt. 2
(1623) II.
i. 98
A fall off of [ 1594 Falling off on] a Tree. 1667 A. MARVELL Corr.
in Wks.
(1875) II.
224
The Lords and we cannot yet get off of the difficultyes risen
betwixt us. 1678 J. BUNYAN Pilgrim's Progress 49 About a furlong off of the Porters Lodge. 1712
R. STEELE Spectator No. 306. 6, I could not keep my Eyes off of her. 1720 D.
DEFOE Mem.
Cavalier 281,
I had perswaded him off of that. 1748 S. RICHARDSON Clarissa V. xiii.
132
Biting my lip, [was to indicate] Get off of that, as fast as possible.
1775 P. OLIVER in T. Hutchinson's Diary 7 Dec. I. 581
A Rebell Pirate..taken..off of Cape Ann.
a1805 in F. J. Child Eng. & Sc.
Pop.
Ballads (1894)
V. IX.
106/2
Aff o
the weather [read wether]
he took the skin,
An rowt his bonny lady in. 1824 J. WIGHT Mornings at Bow St. 21 Two young men..were charged by a watchman with having ‘bother'd him on his bate,’ and refused to ‘go along off of it when he tould 'em.’
1843 T. C. HALIBURTON Attaché 1st Ser.
II.
xii.
210
The groom has stole her oats, forgot to give her water, and let her make a supper sometimes off of her nasty, mouldy, filthy beddin'.
1868 HARTLEY Clock Alm.
in Leeds Mercury Weekly Suppl.
(1895)
5 Oct., He connot forshame
To lift up his een off
o' th' graand. 1875 P. BROOKS New Starts in Life viii.
129 If you could have filled his pockets with gold, and feasted his hunger off of silver dishes. 1884 ‘M. TWAIN’
Adventures Huckleberry Finn vi.
45
I'd borrow two or three dollars off of the judge for him. 1909 G. GREIG Mains's Wooin' 6
He's swallowed the dictionar', min,
an's tryin' to get 'er aff
o's stammack.
a1922 T. S. ELIOT Waste Land Drafts (1971) 5
The reputation the place gets, off of a few barflies. 1962 F. NORMAN Guntz i. 15
, I got hold of this very very old typewriter off of a friend of mine. 1974 J. STUBBS Painted Face xxiii.
284 Get off of me, will you, sir? 1990 B. ROCHE Poor Beast in Rain II.
i. 44, I was hangin' around here all the time, gettin' sweet nothin' off of you.
This may perhaps be more common in American English nowadays, but it certainly didn't originate there.
english.stackexchange.com/users/449/Kosmonaut
I don't know how you can explain to people in general that it's grammatically incorrect, but here is one idea: when you go to a James Brown tribute concert and the singer says "Get up offa that thing" what you could do is to trounce the security guards, climb on to the stage, pounce onto the singer, grab the microphone and say "In fact you should be saying not 'get up offa that thing' but 'get up off that thing' since 'offa' is grammatically incorrect.
" I'm sure everyone will thank you for your grammatical corrections.
Being serious for a moment though, it's very difficult to decide what is or is not "grammatically correct".
E.g. in the UK we say "outside my house" but US people often say "outside of my house".
How can you actually decide which of these two is grammatically correct?
Unfortunately it's not possible since English grammar is not a science like physics but merely a description of what people actually do, and that has changed very significantly over the years.
If I was teaching someone English
I would definitely say "off" is correct and "off of" is wrong, but this kind of decision is based on intuition, based on the usages of educated native speakers.
english.stackexchange.com/users/None/
Off of does sound barbarous to the speaker of British received pronunciations but it's common (in every sene of the word) in regional dialects such as Cockney.
Perhaps grammar is a tribal mark of affiliation rather like the choice of vocabulary as in for example "toilet" vs "lavatory".
english.stackexchange.com/users/None/
The simple answer?
It is an Americanism similar to saying "Don't forget to write me" (American) as opposed to "Don't forget to write to me" (everywhere else).
english.stackexchange.com/users/250/soutarm
Or is there perhaps some simple way I can explain this to most people?
Yes, by explaining that the word of in that context, is unnecessary.
It is superfluous.
An unnecessary added extra.
It is surplus to requirements.
It adds nothing to the meaning of sentences.
english.stackexchange.com/users/19290/Tristan
How can I explain to people that the phrase off of is grammatically incorrect? I‘ve heard this phrase used a lot, especially by Americans (though they aren't the only ones). In my understanding, off of should usually be replaced by off , as in, I took the book off the shelf as opposed to I took the book off of the shelf Am I wrong? Or is there perhaps some simple way I can explain this to most people?
How can I explain to people that the phrase “off of ” is grammatically incorrect?
english.stackexchange.com
<grammaticality><prepositions><phrasal-verbs><off-of>
english.stackexchange.com/questions/619
english.stackexchange.com/users/255/a_m0d
The answer to this question will be subjective, but it's quite possible that the form "off of" is grammatically correct and there are thousands of examples of it in the Corpus of Contemporary American English. It's actual used may depend on region. However, one way to convince people not to use "off of" would be to point out that the word "of" in such a context is unnecessary. Views vary - "off of" is commonly used and there is no rule to say it is incorrect, but at the same time the "of" is superfluous as it adds nothing to the meaning and in that sense it can be said to be incorrect.
The answer to this question will be subjective, but it's quite possible that the form "off of" is grammatically correct and there are thousands of examples of it in the Corpus of Contemporary American English. It's actual used may depend on region. However, one way to convince people not to use "off of" would be to point out that the word "of" in such a context is unnecessary. You can explaini hat the word  'of'  in that context, is unnecessary and superflous, although there are examples of “off of” in the  Corpus of Contemporary American English. “Off of” is well-established as standard in American English, but not used in standard British English.
Although “off of” is well-established as standard in American English, plain “off” may be stylistically preferable in many cases, as "off" can be considered unnecessary in this context. The use of "off of" is, in fact, not grammatically incorrect. As a result, there is no way to explain that people that it is. However, if you wanted to convince people not to use it, you could simply explain that it is unnecessary.
It's not incorrect. "Off of" as a form dates back 500 years and is not incorrect. There is no rule on this. "Off of" is more colloquial in British English. It is an Americanism. The word "of" is unnecessary in this context - it adds nothing to the meaning of the sentence and that is why it is incorrect.
 There are examples of “off of” in the  Corpus of Contemporary American English. “Off of” is well-established as standard in American English, but not used in standard British English. You can explaini hat the word  'of'  in that context, is unnecessary and superflous.
It is not possible to explain to people that the use of "off of" is incorrect because it is, in fact, not. If you wanted to convince people not to use "off of", you might consider pointing out that it is unnecessary.