Starting is often harder than continuing. Jumping into an ongoing conversation is simpler than initiating one, just as expanding on what you already know is easier than learning from ground zero.
For instance, knowing French helped me learn Portuguese in Brazil without formal lessons. In contrast, despite studying German for eight years in school, I struggle to converse in it.
In my private classes, I emphasize building on what students can apply right away. It's counterproductive to introduce concepts they're not ready for. For instance, it doesn't make sense to teach a student how to execute a rear naked choke if they spend all their time on defense during sparring.
Of course, there will be instances where we must start from scratch. Even then, it's rarely from complete ignorance. Patience is crucial. Layer your knowledge, concept by concept. Truly understanding new ideas often requires time, be it months or even years.