1. The adverb bad is not as old as the adverb badly. The OED notes a couple of instances of the former from the 17th century; the OED Supplement picks it up from the early 19th century, calling it "chiefly U.S." But it must have existed in 18th-century British English, because Baker 1770 complains about it:
Some writers employ the word bad as an Adverb, and would not scruple to say That was done very bad: which is not English.... bad is only an Adjective. The Adverb is badly.
In modern use the adverb bad falls into two general areas of use, one of which is standard, and the other of which sounds more like a mistake and is usually considered less than standard.
The standard use of the adverb bad is equivalent in meaning to badly. It often occurs with off:
The Americans didn't know how bad off they were until daylight —E. J. Kahn, Jr., New Yorker, 13 June 1953
Are living composers really that bad off? —Ned Rorem, NY. Times, 20 Apr. 1975
After do and a few other verbs, bad is interchangeable withbadly:
... so I didn't do too bad —Denny McLain, quoted in Sports Illustrated, 29 July 1968
The revenge Watson sought was not against any person, of course, but against Winged Foot itself, where the rule is: Hit it bad and it'll eat your lunch —J. O. Tate, National Rev., 24 Aug. 1984
A bad moving sow will soon go off her legs, and then you must look out for trouble —E. Walford Lloyd, Pigs and Their Management, 1950
Bad is also interchangeable with badly after want or need:
... the war which he says will surely come, though Prussia wants bad to dodge it —Henry Adams, letter, 9 Apr. 1859
... while we continually shout for peace and security, we don't seem to want it bad enough to prepare adequately —Brig. Gen. Robert C. Dean, quoted in Springfield (Mass.) Union, 15 May 1953
... needed dough, and he needed it bad —James Atlas, N.Y. Times Mag., 9 Sept. 1979
I wanted to get a mandolin real bad —Bill Holt, quoted in Bluegrass Unlimited, 11 May 1982
You should note that most of these examples are from speech.
When bad functions in an intensive sense, more or less equivalent to severely, it sounds wrong to more people and is less likely to be considered standard, though Evans 1957 believes that this use will eventually become standard. Here are a number of examples. You will find that some of them are less jarring than others:
... letting himself sink back down into the luxurious willess [sic] irresponsibility that is the nicest thing about being bad sick —James Jones, From Here to Eternity, 1951
... was only a Trotskyist, and hated Communists bad —G. Legman, The Fake Revolt, 1967
He has had frozen feet pretty bad —Walt Whitman, Brooklyn Eagle, 19 Mar. 1863
".. .And the Russians use twenty-five per cent of their population in food production and screw it up so bad they have to buy from the United States...." —Len Deighton, Spy Story, 1974
If the ratings fall a point or two, how bad can you hurt? —Jerry Solomon, quoted in Forbes, 14 Feb. 1983
2. Vizetelly 1906 and MacCracken & Sandison 1917 (and many others of that era) warned against using badly after want in the sense of "very much." By 1958 Bernstein was correcting bad after want to badly, indicating that badly was fully acceptable in this use.
I want to live here badly —Jodie Foster, quoted in TV Guide, 11 Mar. 1983
As we have seen, bad is frequent in this use, but most of our evidence is from speech.
Similar use of badly in the expressions "badly in need of and "need badly," although criticized by commentators in the past, was found acceptable by the majority
of the usage panels of Heritage 1969 and Harper 1975, 1985. It is standard.
I told all my people to go home and get some sleep, which they needed very badly, having missed one night entirely —Dean Acheson, quoted in Merle Miller, Plain Speaking, 1973
3. The use of badly after a copula (or linking verb) is widely discussed, with many handbooks warning of divided usage or warning against the use of badly. Badly comes most frequently after the verb feel, and for the subtleties of that usage, you should see feel bad, feel BADLY.
Our evidence shows badly less common after other linking verbs. The examples that follow all come from standard usage; badly is considered an adjective in this construction:
Henry looked badly —Francis Hackett, Henry the Eighth, 1929
If a body of water is muddy, or otherwise discolored, or smells badly ... we can regard it as polluted — George S. Hunt, Bioscience, March 1965
The stuff tasted badly —Stephen Nemo, Avant-Garde, March 1968
Even though dictionaries recognize this standard use of badly as an adjective, our relatively spare evidence suggests that most writers use bad instead.
4. Evans 1957 notes that the comparative and superlative of bad and badly are worse and worst. At one time badder and baddest were used, but they dropped out of the standard language in the 18th century, surviving only in dialectal use. In recent time badder and baddest have been revived, but only in relation to the slang sense of bad meaning "good, great."(资料出处：韦伯斯特英语用法词典 )