Two different words, two different sounds
"Loose" (rhymes with "goose") means not tight, not restrained, or not precise. "Lose" (rhymes with "choose") means to misplace something, to fail to win, or to be deprived of something. Despite looking similar on the page, they are pronounced differently and have no overlap in meaning.
The confusion is purely a spelling error, not a grammar error. Writers who mix them up usually know the difference in speech but type the wrong number of O's.
How to remember which is which
Think of the double O in "loose" as a pair of open loops, wide and slack, just like something that is loose. "Lose" has lost one of its O's, which is fitting for a word about losing things.
Another approach: "loose" has the same double-O as "goose" and "moose," which helps lock in the pronunciation. "Lose" has the same pattern as "chose" and "whose."
- The bolts are loose and need tightening. (not tight)
- Do not lose your parking ticket. (misplace)
- She wore a loose-fitting jacket. (relaxed fit)
- We cannot afford to lose this client. (fail to retain)
Editing tips for catching the error
Because spell-checkers will not flag this (both words are valid), you need a manual check. Search your document for "loose" and verify each instance means "not tight." Then search for "lose" and verify each instance means "misplace" or "fail to win." This targeted search takes under a minute and eliminates the error entirely.
"Loosing" is a real word (meaning to set free or release, as in "loosing an arrow"), but it is rare. If you have written "loosing," you almost certainly meant "losing."