I have read a sentence from “Diary of a Wimpy Kid 4”:
The teachers have really been cracking down on kids copying off of each other this year.
How to understand "off of"? Please parse it.
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You get into a box. (Although you can also get in it, which means the same thing.) Once you have gone into it, you are then in it.
Similarly, you can get out of a box. Once you have gone out of it, you are then outside it.
In the U.S., off of and off correspond to into and in. You can jump off of the roof. (You can also jump off it; this means the same thing.) Once you have jumped off of it, you are then off it (but maybe you need to go into the hospital; don't try this at home).
The phrase "off of" was used in the U.K. in exactly the same way before 1750 or so; for example, in 1724 Daniel Defoe wrote
and with that he flung himself off of his Horse.
If you look at Google Ngrams you can see a remarkable drop in the frequency of "off of" in English between 1740 and 1780. It's rebounded more in the U.S. than in the U.K. Possibly some authority declared that it was redundant or ungrammatical in the early 18th century, and people started avoided it in formal writing. Or (since I can't find any such authority) maybe this was just how the language evolved in England.
There seem to be some people who seem to have the idea that off of is a recent abomination that has polluted the English language, and that you should only ever use off or from. Remarkably, considering that off of is more common in the U.S., a considerable fraction of these people are Americans. I think these people are wrong, and should be ignored.
There are two things to be aware of.
1) To copy off of someone is a phrasal verb meaning that you're copying content from their paper. This has a different meaning from simply to copy, which can be to copy someone's information from anywhere (from online, from a book, and not necessarily from their paper). The way it's used usually implies that the person is with their work at the time. To copy off someone (without of) is also common and carries the same meaning.
2) Off of versus off: Few people have retained this awareness, but off of is the correct preposition to use with a transitive verb, as in
We jumped off of the boat and into the water.
Many today would simply say
We jumped off the boat and in the water.
which is technically incorrect unless we jumped (around) while we were on the boat and also while we were in the water.
According to Google Ngram Viewer: 1) "off of" is about twice as common in American English as in British English.
2) "of" is the 10th most common word to follow "off" in American English and slightly lower than the 10th most common in British English. (There are some other differences in words and orders.)