What to Know
Dreamed and dreamt are both acceptable past tense forms of dream. Dreamed follows the pattern of regular verbs, ending with "-ed" while dreamt is irregular. Often the irregular, or "strong," form of a word gives way and is replaced by the normalized form, but both dreamt and dreamed are still in use.
Let's say it's Monday morning at the water cooler and your coworker is recounting another one of their fascinating dreams: "It was cold, and we were all walking across a big field, and there were fish swimming around our feet even though there wasn't any water, and …" It can be hard to stifle a yawn, can't it?
We're not going to weigh in on whether or not other people's dreams are always boring (of course they're not!), but we will weigh in on what the past tense of dream is.
What was it your coworker did last night? They dreamed about that cold fish-filled field? Or is it more correct to say they dreamt about it?
The answer is either.
Both dreamed and dreamt have been past tense forms of dream since the 14th century. "I dreamt a dream tonight," says Romeo to Mercutio in Shakespeare's Romeo and Juliet, written in the late 16th century. Shakespeare typically opted for dreamt in his works, but occasionally employed dreamed as well. A century and change later, Jonathan Swift vacillated between dreamed and dreamt in Journal to Stella, a series of letters written between 1710 and 1713 and published posthumously in 1766, but chose dreamed for the one past-tense occurrence of dream in the 1726 Gulliver's Travels. By the 19th century, evidence suggests that most major writers (or perhaps their editors and/or publishers) were somewhat conflicted. While Jane Austen and William Makepeace Thackeray were dedicated dreamt users, and Edith Wharton and Virginia Woolf consistently favored dreamed, other 19th and early 20th century writers—among them Charlotte Brontë, Mark Twain, G.K. Chesterton, Herman Melville, Walter Scott, Joseph Conrad, Jack London, Charles Dickens, Thomas Hardy, Henry James, H.G. Wells, James Joyce, and P.G. Wodehouse—used both. But both the literary world and English speakers generally were moving decidedly away from dreamt, with dreamed becoming the clearly dominant form in the first half of the 19th century.
Regular and Irregular Verbs
Dreamed, of course, follows the pattern of most verbs. The great majority of English verbs take the familiar -ed for their past tense and past-participle forms. These are "regular" verbs that play by the rules. Other not-so-predictable verbs are "irregular." The regular verbs are sometimes called "weak" and the irregular verbs sometimes called "strong," presumably because the former are a docile and tractable bunch while the latter seem to do whatever they gosh darn well please. Merriam-Webster's Advanced Learners English Dictionary (a dictionary for non-native speakers) lists about 300 irregular verbs, the majority of them being simple, usually single-syllable words. It's a small number, but its members are powerful: they include those we use most often; as linguist Steven Pinker has pointed out, the ten most common English verbs (be, have, do, say, make, go, take, come, see, and get) are irregular, and chances are quite good (70% good) that if you're using a verb it's an irregular one.
Both regular and irregular verbs date back to Old English, but the number of ho-hum -ed forms has increased over the centuries, and only the most common irregular verbs have kept their quirky conjugations. There are still glimpses of the less common strong verb forms here and there, especially in dialectal English. Someone native to parts of the South might say "I love to climb trees but have never clomb/clome that one there." Climbed has been the norm since around the 16th century, but the other form still exists, secreted away in dialects.
Every once in a while things go in the opposite direction. Sneak had the regular past tense form sneaked when it appeared in the late 1500s, but in the late 1800s the form snuck showed up in the United States. That form is now more common here by some estimations than sneaked. Snuck is widely derided in the UK—but still used there some, and in respected newspapers—and it's got its haters on this side of the pond too.
Although dream appears to be yet another verb that has followed the expected trajectory of weakening into regularity, both dreamed and dreamt are in current use, and you can use the stronger and less common form if you prefer it. We wouldn't dream of telling you otherwise.