1. The OED traces leave "let" in the com­bination leave alone back to about 1400. This leave alone, however, does not seem to have been a very con­spicuous part of the literary mainstream. Shakespeare, Congreve, and Defoe, for instance, use leave alone only in its literal sense of "leave in solitude"; they use let alone for "to refrain from bothering or using." Shake­speare, for example, has this:

So please you, let me now be left alone —Romeo and Juliet, 1595

It seems likely that leave alone "let alone" did not begin to appear in print often until the later 19th century (the OED has a 1798 example from Madame D'Arblay's diary, but it was not published until nearly 100 years later). This use was familiar in one special context early in the 19th century:

Little Bo-Peep has lost her sheep,
    And can't tell where to find them;
Leave them alone, and they'll come home,
    And bring their tails behind them.

The Oxford Dictionary of Nursery Rhymes (1952) dates this back to an 1805 manuscript. It appeared in print in editions published in 1810, 1842, 1853, and 1877 and undoubtedly did much to make the phrase familiar. We have another early 19th-century instance, but it too would not have been published until much later than it was written:

... and heartily pray that they would leave me alone —Charles Lamb, letter, 29 June 1829

Leave alone and other phrases using leave in the sense of "let" (the OED lists leave go (of) and leave hold (of) among others) must have begun to appear with some frequency by the last quarter of the 19th century, for that is when they first drew unfavorable notice, from Ayres 1881. He condemns such expressions as "leave me be" and "leave it alone" as vulgarisms. Compton 1898 and Vizetelly 1906 agree and so does Bierce 1909. Bierce is apparently the first to mention the "solitary" versus the "untouched" sense of leave alone.

Leave alone was not singled out for special treatment by commentators until sometime after World War II. Commentators in the 1920s and 1930s continued to lump it with other leave expressions when they men­tioned it at all. Occasionally, as in the Literary Digest of 24 January 1925, it was contrasted with let alone along the "solitary"-"untouched" lines of Bierce. Since World War II, opinion has been divided between the insisters on the Biercian distinction (Copperud 1960, 1964, 1970, 1980 and Bernstein 1958, 1965, 1977) and just about everyone else (including Evans 1957, Bryant 1962, Watt 1967, Shaw 1975, 1987, Harper 1975, 1985, and Trim­mer & McCrimmon 1988), who find let alone and leave alone acceptably interchangeable. Watt makes the point that the distinction between leave alone and let alone can be very slight, and it is hard to get exercised over the possibility of ambiguity—as Bernstein and Coppe­rud do—in an example like that from Charles Lamb, which could be interpreted as "leave me by myself or "don't bother me." Lamb's drift is unmistakable no matter which interpretation you choose.

F. Scott Fitzgerald had noticed the phrases being used interchangeably:

"Then you ought to leave it alone," countered Doc­tor Civet — The Great Gatsby, 1925

"Why not let her alone, old sport?" remarked Gatsby —The Great Gatsby, 1925

Some other examples of leave alone:

... your imagination. I say, leave it alone, let it do its own work —Hal Underhill, The Writer, May 1968

But a good many other Americans at that time could take a college education or leave it alone —Tom Wicker, Change, September 1971

My mother hated whisky and admired men who could leave it alone —Russell Baker, Growing Up, 1982

You should note that the "solitary" sense can still be used unmistakably when it is wanted:

... she was afraid that she would be left alone. Once you have lived with another, it is a great torture to have to live alone —Carson McCullers, The Ballad of the Sad Cafe, 1951

Leave alone is standard in both uses.

2. Many of the American commentators on leave and let—particularly those who write schoolbooks and col­lege handbooks—make no mention of leave (or let) alone but concentrate on other uses of leave meaning "let." The construction usually condemned is leave fol­lowed by a pronoun and an infinitive without to. Leave in this use is often a mild imperative:

Leave us only add that ... —Jonathan Evan Mas-low, Saturday Rev., 12 Apr. 1980

This construction is primarily oral. It has been so long and so often reprehended in the schools that one of its chief uses in print is to be facetious:

Leave us not enquire how I happened to be running —Alan Coren, Punch, 7 Oct. 1975

When it is used seriously in print, the use has been taken from speech:

If you put them in drums, and this is the easiest way to handle them, and you leave them stand, some free-liquid will surface over time —John Hernandez, quoted in Civil Engineering, December 1982

As for the gap left by Sweet, "We will probably leave it be," says NBC casting chief Joel Thurm —TV Guide, 31 May 1985

One curious thing about these constructions is that they are held to be more reprehensible in American English than in British English. They have received almost no attention by British commentators, as far as we can tell, until quite recently, when Phythian 1979 and Longman 1984 take notice. Perhaps British indif­ference accounts for occasional serious (but not literary) use in British publications:

Leave it steep for at least 4 hours —Elizabeth David, Italian Food, 1969

This example points out how minor the problem really is. In American English, "Let it steep" or "Leave it to steep" would both be acceptable. But no matter how you choose to define let and leave here, the same message has been given.

There are other idioms in which leave and let are involved, notably the OED's leave go (of) and leave hold (of). About these American commentators are in disar­ray, and the only British commentator to mention them, Phythian 1979, disapproves leave go (of). The OED simply identifies them with spoken—colloquial— idiom.

What conclusions can we draw? In American use, let plus the infinitive without to and leave plus to plus the infinitive are standard and acceptable to all parties. Leave without to but with the infinitive is monolithi-cally opposed by American educational usage writers, still devoted to the opinion of Ayres 1881. It is, conse­quently, a construction limited to speech and to face­tious use in print. British attitudes seem more latitudi-narian, or perhaps just indifferent.

3. The last curiosity about leave and let to be mentioned here is the substitution of let for leave, which is dis­cussed in Bryant 1962. Bryant's examples are dialectal, and were recorded in areas of Pennsylvania Dutch set­tlement. An example is "Don't close the door; let it open." A German verb lassen, which overlaps both leave and let in meaning, is supposed to be the influence behind the usage. H. L. Mencken in The American Lan­guage, with some confirmation by Raven McDavid, edi­tor of the 1963 abridged edition, also mentions German as an influence in the confusion of leave and let. Here is a fictional example of the dialectal confusion:

"Please, will you leave me let my books in school? ..." —Helen Reimensnyder Martin, Tillie—A Men-nonite Maid, 1904

We might mention that hypercorrection seems to have reared its less than charming head in this matter. A well-known New York baseball announcer, who is distinctly not of German background, has been recorded on at least two occasions as saying "He's gonna let him in" when a baseball manager, after a visit to the mound, has decided to stick with the pitcher currently in the game. Most of us would say "leave him in." The let is presumably the result of frequent teacherly correction of leave constructions.(资料出处：韦伯斯特英语用法词典)