I think this could be better expressed with the conditional here:
If he hears it, he will forget it.
If he sees it, he will remember it.
If he does it, he will understand it.
Chinese often has an "implied conditional" in this kind of "classical-maxim structure", even though this conditionality may not be overtly stated (as in speech in more conversational, modern Chinese). ---
Actually, I have heard a similar proverb in English, which I think went something like this:
Tell me, and I'll forget.
Show me, and I'll remember.
Let me try it, and I'll understand... ---
PS: Yaho's comments in Chinese above simply explain and illustrate the meaning of the original text; they are not a comment about the translation itself.