Through zeal knowledge is gotten, through lack of zeal knowledge is lost; let a man who knows this double path of gain and loss thus place himself that knowledge may grow.

Knowledge is not a possession but a rate. What you stop using drains away quietly, and what you attend to keeps growing, so at any moment you are on one of two slopes and rarely on flat ground. The drift is slow enough to be invisible month to month and obvious across a decade. Nobody decides to forget a language or a skill, they simply stop practising it.

Name one thing you used to know well and have let slide. Either give it an hour a week or let it go on purpose rather than by default. Deciding is better than drifting, either way.