Not just languages. It's the case with everything. Easy to spot when you're tutoring someone. You can see they learned "force is mass times acceleration" or "for( ... ) is how you make the same code run multiple times" - they can tell you that when quizzed! But you know they haven't comprehended it until they can reverse it - "I need to compute the mass of the object, which I see accelerating this much under this force; I can pull that from F=ma -> m=F/a!", or "I want to run this code several times, I need a `for` loop!". And getting to that stage is often the longest and most difficult part.