Here's a quick little mneumonic to help you tell the difference: "lose" loses an "o".
Here's why, with examples from Harry Potter (and much assistance and many examples from dictionary.com!).
"Loose" is primarily used as an adjective meaning "free and unrestrained". Here's the definition, with some exciting examples.
- free or released from fastening or attachment: Harry, having finished his charms homework, found himself at loose ends for the evening.
- free from anything that binds or restrains; unfettered: Having escaped from his detention with Snape, Harry put on his invisibility cloak and prowled like a loose cat around the darkened school.
- not bound together: Ginny was accustomed to wearing her hair loose, not pulled back in a braid.
- not put up in a package or other container: Harry went to Hogsmeade with a pocketful of loose change to spend on butterbeer.
- lacking in reticence or power of restraint: Harry had no need to worry that Ron would tell anyone his secrets: Ron didn't have a loose tongue.
- lax, as the bowels. I think this is pretty self-explanatory.
- lacking moral restraint or integrity: Draco Malfoy was notorious for his loose character.
- sexually promiscuous or immoral; unchaste: Hermione was aghast at being called a "loose woman" in the newspapers for her friendship with Harry.
"Loose" can be applied to a wide variety of situations as an adjective, but it always communicates the idea that something is free, not tied up or held down or restrained. Maybe you can guess now at what "loose" means as a verb...
Loose (verb): to let loose; free from bonds or restraint. to unfasten, undo, or untie, as a bond, fetter, or knot. to shoot; discharge; let fly. to make less tight; slacken or relax.
Finding Ron tied up on the floor, Harry immediately set about loosing his bonds.
The last thing Harry wanted to do was to loose the three-headed dog!
Harry ducked for cover as the Death Eaters loosed another volley of magical missiles at him.
There are a number of common idioms using the word "loose", all of which carry that same meaning of "setting free":
- break loose, to free oneself; escape: Harry broke loose from the Death Eater's grasp and ran away.
- cast loose, to send forth; set adrift or free: With his parents' death, Harry was cast loose at an early age to make his own way in the world.
- cut loose / let loose, to revel without restraint: The Yule Ball was a chance for the Hogwarts students to let loose and have a good time.
- on the loose, free; unconfined, as, esp., an escaped convict or circus animal: With Sirius Black's escape from prison, a convicted murderer was on the loose!
- turn loose, to release or free, as from confinement: It was such a beautiful day that Professor Snape turned his potions students loose to frolic in fields of daisies.
Well, that last example might be a little improbable, but you get the idea. When something's loose, it's free! Unchained! Nothing's restraining it from leaping out into the wide world!
To "lose" something, on the other hand, means that something important has been misplaced:
- to come to be without (something in one's possession or care), through accident, theft, etc., so that there is little or no prospect of recovery: Harry was terrified to think he might lose his invisibility cloak.
- to fail inadvertently to retain (something) in such a way that it cannot be immediately recovered: Ron was sure that chocolate frog card he'd lost was around here somewhere.
- to suffer the deprivation of: Harry was determined not to lose his life in a battle with Voldemort.
- to be bereaved of by death: Mrs. Weasley was terrified of losing one of her beloved children.
- to fail to keep, preserve, or maintain: Mrs. Weasley was glad to see that even after Ginny's birth she still hadn't lost her girlish figure.
- (of a clock or watch) to run slower by: Uncle Vernon shook his watch, certain it was losing time--surely Harry should be gone already?
- to give up; forfeit the possession of: Ludo Bagman wouldn't admit to losing a fortune to the Weasley twins.
- to get rid of: Harry had to lose his fear in order to fight the Boggart successfully.
- to bring to destruction or ruin (usually used passively): Thousands of people, Muggles and wizards alike, were lost the last time Voldemort surfaced.
- to have slip from sight, hearing, attention, etc.: Hogsmeade was so busy, Hermione lost sight of Ron in the crowd.
- to allow (oneself) to become absorbed or engrossed in something and oblivious to all else: Harry lost himself in thought as he pondered his Triwizard egg.
- to suffer defeat or fail to win, as in a contest, race, or game: Griffyndor played well in the Quidditch match, but they still lost to Hufflepuff when their Seeker caught the Snitch.
The two verbs "loose" and "lose" are often confused--it's easy to stick an extra "o" in there! But think it through when you need to use one of these words: "loose" means to set free and to be unrestrained, and "lose" means to misplace. Harry might lose his wand, but he would never loose it! Or, to borrow an example from above, if Mrs. Weasley worried about loosing one of her children, it'd mean she had them chained up in the basement. And she's not that kind of lady! You can see how this error can change the meaning of what you're trying to say.
Changing the sentence to past tense might help you remember which word to use. The past tense of "loose" is "loosed", and the past tense of "lose" is "lost". "Loosed" and "lost" sound and look more different than "loose" and "lose", so that may help you know which is correct. Also keep in mind that "loose" is related to "loosen". If the meaning of the sentence is related to something being "loosened", then you'll need the second "o".
Spellcheck won't catch this error, so remember: "loose" means "to set free", "lose" means "to misplace".
Or here's a terrific mneumonic from theemdash: If you lose the second "o," you've let loose a letter.
This post relies heavily on assistance from the dictionary.com definitions of "loose" and "lose".