The phrase all but means “almost” or “nearly,” but not quite completely. When something is all but finished, it's so close to done that only tiny details remain. If a race is all but over, the winner is so far ahead that the outcome is essentially decided, even though the finish line hasn't been crossed yet.

This phrase can sound confusing at first because “all but” seems like it should mean “everything except,” but it actually means the opposite: it describes something that's practically complete or certain. If your teacher says the class has all but mastered long division, she means the class nearly understands it. When a detective says a mystery is all but solved, only small pieces of evidence need confirmation.

The phrase works because it means “everything has happened but the final moment.” Think of building a 1,000-piece puzzle: when you have 999 pieces in place, the puzzle is all but complete. You've done all the work except placing that last piece. Similarly, when autumn leaves have all but fallen from the trees, maybe a few stubborn ones cling to the branches, but the season's change is essentially complete.