it is 은/는. 은 is for words ending in batchim (the bottom half of character) and 는 is used for words that don't end in batchim.
If you mean memorize the particle itself, it is used a ton so you genuinely don't need to worry about that. But if you mean memorise it's meaning, you'll just have to look back whenever you forget and look at examples which you don't need to look for.
