Note: Many of the entries here will eventually become full-length posts. Some are rough and have not been fully researched. If you have any corrections or would like to add anything, please comment.
Backstage: one word.
Backwater: an isolated place.
Bad vs. badly: Bad is a long-established flat adverb (i.e., an adjective that can become an adverb without adding -ly), but many people believe using bad as an adverb is always wrong. You can either ignore these people or play it safe by using badly.
Bailiwick: a person’s field of interest or expertise.
Bane: a cause of harm, fatal injury, or death.
Baptise vs. baptize: North America: baptize, baptized, baptizing. Outside North America: baptise, baptised, baptising. But baptism is used everywhere.
Bare [one’s] soul: Bare here is in the sense uncover or reveal. Bear one’s soul is a misspelling.
Barely: bearly is not a dictionary-recognized word.
Bastion: a defensive stronghold—literal or figurative.
Bath vs. bathe: Bath is the noun. Bathe is the verb. When you take a bath, you bathe yourself.
Battle royal: a battle involving many fighters. Not battle royale, unless you’re talking about the Japanese novel and movie by that name.
Bear in mind: keep in mind. Bare in mind is a misspelling.
Beat around the bush: to talk around a subject without addressing it directly.
Beat -> beat -> beaten: Beat is the present and past tense. Beaten is the past participle. E.g., We beat them yesterday, but they have beaten us before.
Bed rest vs. bedrest: usually two words, but the compound is gaining ground. Bed-rest appears occasionally.
Befuddle: formerly, to make stupid with drink; in modern usage, to confuse.
Began, begun: Began is the past tense (we began yesterday). Begun is the past participle (we have just begun).
Begrudge: synonymous with envy.
Beholden: owing to another. When you are beholden to someone, you are indebted to that person.
Behoove: to be necessary and proper. It does not mean to benefit.
Belle: a beautiful woman or girl. Belle is the correct spelling in belle of the ball, Southern belle, and related phrases.
Bended vs. bent: Bended is the original form, but bent has prevailed for several centuries. Now, bended mostly appears in the phrases on bended knee and on bended knees.
Benefactor vs. beneficiary: The benefactor provides benefits. The beneficiary receives benefits.
Benefit: makes benefited and benefiting—one t—in all varieties of English. Benefitted and benefitting were once more common than they are now, but the one-t forms have always prevailed.
Beside oneself: in a state of extreme emotion.
Bespeak: to indicate, to give a sign of, or to address.
Bespectacled vs. spectacled: Spectacled is perfectly good, but people like to say bespectacled. Spectacled was once the more common form, but the longer word has been ascendant for the last several decades.
Bill of goods: a dishonest or misleading plan or promise.
Bingeing vs. binging: In the U.S., binging is binge‘s present participle. Binging appears outside the U.S. as well, but bingeing is more common.
Bite-size vs. bite-sized: Bite-sized may seem logical, but bite-size is standard.
Black out vs. blackout: Black out is a verb. Blackout is a noun and an adjective.
Blew, blown, blowed: Blew is the past tense (I blew on my soup). Blown is the past participle (I have blown on it for an hour). There is no reason ever to use blowed.
Blitzkrieg: from German for lightning warfare, coined for the Nazi style of swift, overwhelming attack. Today it is often used figuratively for any swift and intense attack.
Body politic: the people of a politically organized nation.
Bogeyman: Bogeyman is the original most common spelling of the noun referring to an imaginary figure that scares children.
Bone of contention: a small matter preventing agreement.
Bone to pick: a small matter constituting grounds for complaint or dispute.
Boondocks: backwoods, or a remote, rural area. Sometimes shortened to boonies.
Bough vs. bow: A bough is a tree branch. Bow has no definitions relating to branches.
Bounteous vs. bountiful: no significant difference between them. Both mean (1) giving generously, or (2) plentiful.
Brackets: Brackets, [which look like the marks surrounding this clause], are used for parentheticals within quotes and within other parentheticals.
Braggadocio: (1) a person who is wont to brag in a self-inflating manner; (2) empty bragging.
Braggart vs. bragger: Use braggart for a habitual bragger. Bragger is occasionally useful for people who are not habitual braggers.
Brain trust: a group of experts serving as advisors. Two words.
Brake vs. break: Brake is the word for mechanisms that stop motion and for reducing speed with such a mechanism. Break has no definitions relating to stopping motion or slowing speed.
Brand spanking new: very new. Spanking here is used in an old, now seldom-used sense—i.e., very big or exceptionally good.
Bravado: (1) swagger, or (2) a pretense of courage.
Brief vs. debrief: To brief someone is to give them a summary or important facts, especially in preparation for something. To debrief someone is to gather knowledge from them, often in an official or classified manner. The word is usually used in military contexts.
Bring vs. take: You bring something to something. You take something from something.
Bromide: metaphorically, a platitude or a trite notion.
Bugbear: an object of dread, especially one that is imaginary. In its original sense, bugbear denoted an imaginary bearlike creature invoked to scare children.
Bum rush: (1) to hurry someone out of a place, and (2) to get a decisive early advantage with a quick, possibly unfair, move.
Bunny: It is so spelled in all its senses. Bunnie is only a name.
Byproduct: one word.