You read a news headline and 心配 (worry) gives you no trouble at all. You know it, you know how it's read, you know what the sentence means. An hour later you want to tell someone you're a little worried about a deadline, and nothing comes out while you search for a word you were sure you knew. It was 心配, the same word you read without a second thought an hour earlier.
If this sounds familiar, nothing has gone wrong with your studying. You are running into the gap between two vocabularies that every language learner has, and every native speaker too. Once you see how it works, it becomes much easier to do something about it.
Two vocabularies, not one
Your receptive vocabulary, sometimes called passive vocabulary, is the set of words you understand when you read or hear them. Your productive, or active, vocabulary is the set of words you can bring to mind yourself and use when you speak or write. Japanese has its own pair of terms for this: 理解語彙 (vocabulary you understand) and 使用語彙 (vocabulary you use).
It helps to think of these less as two separate lists and more as two depths of knowing the same word. The researcher Paul Nation, whose book Learning Vocabulary in Another Language is a standard reference on the subject, breaks knowing a word into many parts: its spoken and written form, its meaning, and how it is used. Each part has a receptive side and a productive side.
Take 促す (to urge or prompt). Knowing that it means "to urge" when you meet it in a sentence is receptive knowledge. Producing it means pulling up うながす from a blank, writing it as 促す, putting it in the past as 促した, and knowing that what someone is urged to do comes before it with 〜よう or 〜ように. That is a lot more to have ready.
Why the gap exists
Recognition gets help; production starts from nothing. When you read, the sentence around a word, its kanji and the topic all narrow down what it could mean. When you speak, you start with an idea and have to find the exact word on your own, quickly, with someone waiting.
Fuzzy knowledge is enough to recognize a word, but not to produce it. You can recognize 心配 from a rough memory of its shape. To produce it you need the whole thing: every kana of the reading, the right kanji, and the right ending if it's a verb or adjective.
Most of what you take in is receptive. Reading, listening, watching shows and reviewing cards with the Japanese on the front all practice recognition. Many learners get far more of that than they get practice producing words.
Native speakers have the gap too. Everyone understands words they would never think to use. And everyone knows the feeling of a word being right there and not coming out. Japanese describes it as 喉まで出かかっている, roughly "it has come up as far as my throat."
What the research says
Researchers in second language vocabulary have studied this gap for decades. The details vary from study to study, but a few findings come up again and again.
Receptive vocabulary is larger, and the gap is biggest for less common words. Stuart Webb tested Japanese university students learning English with matched receptive and productive translation tests, and published the results in Studies in Second Language Acquisition in 2008. Receptive knowledge was larger than productive knowledge, and the difference grew for lower-frequency words. How large the gap looked also depended on how strictly answers were scored, for example whether a misspelling counted, so no single ratio applies to everyone.
Production grows more slowly. In a 1998 study in Language Learning, Batia Laufer and Tahereh Paribakht compared learners' passive and active vocabularies in English. Active vocabulary grew more slowly than passive vocabulary, and the gap tended to widen as learners knew more words. The setting mattered too: how much of what learners understood became active depended in part on whether they were learning in a classroom abroad or living in an English-speaking environment.
A sentence with a gap is a good way to test production. Laufer and Nation designed a test of what they called controlled productive vocabulary (Language Testing, 1999). Each item is a sentence with a missing word, and the first few letters are given so that only one answer fits. The learner has to produce the word, but the sentence supplies enough context to make the task fair. That design is worth remembering, because it works as practice as well as a test.
You get better at the direction you practice. In a 2009 study in RELC Journal, Webb had Japanese learners of English study word pairs in one direction or the other. Learning from the native-language meaning to the English word (productive learning) built more productive knowledge, and it also helped with spelling. Learning from the English word to its meaning (receptive learning) did more for recognizing meaning. The direction of study shaped what kind of knowledge people ended up with.
Why this matters for how you study
Speaking runs almost entirely on productive vocabulary, under time pressure. If your practice is mostly recognition, you can build a large, real vocabulary and still find that very little of it is available when you open your mouth.
The most common culprit is the one-way flashcard. Japanese on the front, English on the back: you see 心配, think "worry," flip, and mark it correct. That is useful work, but it trains exactly one direction. It never asks you to start from "worry" and arrive at しんぱい.
There is also good evidence that the act of pulling something out of memory is what makes it stick. In a well-known 2008 study in Science, Jeffrey Karpicke and Henry Roediger had students learn Swahili–English word pairs. Students who kept being tested on the pairs remembered far more of them a week later than students who kept restudying them, even though the groups looked similar by the end of the learning session. Retrieval is the exercise. The question is which retrieval you are exercising.
How to close the gap
You don't need to make every word productive. You need the words you actually want to say to be ready when you want them. These habits help.
1. Practice the other direction
Start from the meaning and produce the Japanese. If you use flashcards, add the reverse card, or cover the Japanese and say or type the word before you look. Be strict with yourself: "I'd know it if I saw it" is a receptive answer. The productive answer is the exact word, reading and all.
2. Retrieve words inside sentences
Isolated words are a hard and slightly artificial target. A sentence with a gap is closer to real speaking, because the context tells you what's needed and you still have to supply the word:
彼は私に早く帰るよう＿＿＿。
(He urged me to go home early.)
The answer is 促した: not the dictionary form 促す, but the form the sentence needs. Gapped sentences make you produce the word, its form and its fit all at once. You can make your own by blanking out a word in example sentences you've already studied, then coming back to them a day or two later.
Typing the answer, rather than just thinking it, keeps you honest. It's easy to believe you'd have got a word when you only half had it.
This is how Tangoya's Recall question works. It shows a Japanese sentence with a gap and you type the missing word, in kana or kanji, as it's inflected in that sentence.
3. Produce words about your own life
Choose a few words you understand but never use, and use each one on purpose today. Write three sentences about your week, send a message to a language partner, or talk to yourself while you cook: 独り言 (talking to yourself) is free speaking practice with a very patient listener. The linguist Merrill Swain argued in her output hypothesis that producing language pushes learners to notice what they can't yet say. Those moments where you reach for a word and come up empty are exactly the words to look up and practice next.
4. Choose what to make productive
Productive knowledge takes more work per word, so spend it where it pays. Common, everyday words come first, since you'll need them in nearly every conversation. A word you only meet in news articles can stay receptive for a long time without costing you much. Think about register as well: if you want to chat with friends, casual words matter more than the formal ones you see in announcements.
5. Measure the two sizes separately
A single "words known" figure hides the gap, so it's worth tracking recognition and production apart. Tangoya does this with two numbers, each a share of the 10,000 words most used in everyday conversation: "You understand" and "You can produce." Under them sit four skills. Read (see a word, pick its meaning) and Listen (hear it, pick its meaning) are the understanding side. Recall, which gives the "You can produce" number, and Write (pick which kanji a word you heard is written with) are the producing side. Seeing them side by side makes it clearer where your time should go, and the app acts on it too: two more of each day's new words go to whichever number is lowest, so a lagging Recall gets extra gapped sentences until it catches up.
The gap is a map
Understanding far more than you can say is not a flaw. Every word you recognize is a word that's partly learned, and the receptive side usually comes first. The productive side catches up when you practice it directly: retrieving words from meaning, in sentences, in your own speech and writing.