When learning words in a foreign language (Japanese or any other), it can be frustrating because some words seem to stick immediately whether others take many iterations. While I think some of this just the unpredictability of our memories, it can be instructive to think about what drives this process and to develop ways to help retain words better.
Surely, emotional involvement is an important factor: hearing the cries of someone screaming for help in a foreign language will surely have better odds of being remembered as opposed to reading a dry list of vocabulary words in a textbook. That’s one reason that academic learning only goes so far, you have to get out there and actually use the language––and I feel that in situations where you are nervous or excited, you are actually more likely to remember new words.
The emotional involvement doesn’t necessarily have to come from a live experience. I remember a long time ago when a friend was explaining to me about the word もったいない he explained a story when he was at Disney World with some Japanese friends who said to told him もったいない when somebody dropped an ice cream cone. For some reason, that brief anecdote, together with the word, stayed stuck in my head all these years later.
Another way we can pick up certain words more quickly is if they have some connection with something we already know, whether it be an explicit connection (i.e. a similarity to another word we know) or something hidden in our subconscious. You can use tricks like creating easy-to-remember associations, but while I feel they may have some success, ultimately they are nothing more than a crutch and get in the way; even if they work, they may take longer to go through the connection process than remembering the word or phrase naturally. To give you an example, recently I was frustrated that I couldn’t remember the word for “grading” (like a paper), and so when I looked it up and saw it was 採点 (saiten), I remembered “grading is the opposite of a genius”, which isn’t quite a logical statement but helped me to remember it easily. Having said that, the first few times I tried to use this trick to remember the word it took me a few seconds.
One time I remember hearing the phrase “そりゃそうだけど” (which essentially means “That is true, but..”), and because it was a simple concept I was familiar with––and probably had been wanting to say before in Japanese––it stuck in my head after only hearing it a single time.
One way to learn words is to just try and memorize a list of words, and then when you have an experience, try to match those words up. But I have actually noticed myself learning words in the other direction: having an experience you don’t have a word for, and then later discovering that word. When you do hear it (even if a different context), something clicks and the word isn’t likely to be forgotten soon. I think part of the trick here is the act of trying to match an experience with a word (even if you don’t know it yet) will help wear a “track” in your mind that makes that concept more accessible in the future.
Similarly, if you heard a word and have no idea what it means, rather than immediately looking it up I would suggest trying just to think about it in the context you heard it in. Try to make a guess about what it meant, maybe sleep on it. Then, once you look up the word you’ll have a “eureka” moment where you say “Oh, that is what it means!” and you’ll be more likely to remember the word.
Another technique is to repeat a new word after you have just heard it. This helps with not only remembering the word but also with proper pronunciation (including intonation, which can also be tricky to master). And you can do it even if you don’t know quite what the word means. Sometimes I’ll repeat back what I heard, tagged with ~なんですね just to keep up in the conversation, even if I don’t know 100% what it meant. Of course, if you do this too often the other person might quickly tire of the conversation (:
If you aren’t sure what a word means in the middle of a conversation, but do have a guess, there is nothing wrong with just straight-out asking the other party to confirm your understanding: Ex: 食卓とはご飯を食べるテーブルのことですよね？
Also, while is may not be feasible to do all the time, if you have time after a conversation ends you can try to write down a few of the words to look up later, or just to quiz yourself on the next day.
Kanji, while arguably one of the more difficult parts of the Japanese language, can actually help you memorize new words, or even guess at their meaning just by guessing the characters that the words are written with (oh, if we could only talk in kanji…) This is not a technique I would expect beginners to be proficient at, but it’s another reason to fit kanji study as an important part of your study plan (just don’t force yourself to learn hundreds of kanji in a small period of time!).
Japanese, like English, also has many compound words (ex: 魔法遣い), and that means the more words you learn, when you do come across a new compound word the more likely you’ll be to know one or more of the sub-words comprising it.
A final suggestion is to try and learn at least one word every time you watch an episode of anime, read a chapter of a manga, have a conversation, or read an article. Say it to yourself two or three times when you come across it, and then repeat it again once you are done with that segment. Making a sentence or two with the word will help further cement the word in your mind.