All speakers of probably all languages know a lot more words than they use. I myself have a big vocabulary in English—I can tell you what a lot of words mean, but I might not be able to come up with them on my own to use in conversation. When I was in high school, I became fluent in French. I can still (40+ years later) read it pretty well, but I can barely put a full sentence together to say to a French speaker!
The words you know but can't use well are called your "passive vocabulary". The ones you use easily are in your "active vocabulary".
Passive vocabulary = words you know and recognize but can't use.
Active vocabulary = words you can actually use in speaking.
The problem for non-native speakers is that having a big passive vocabulary in the language you're learning doesn't help you have conversations or participate in discussions. You need to be able to call up a word quickly, right when you need it, or the conversations and discussions will have moved on long before you open your mouth.
The good news is that expanding your active vocabulary doesn't mean you have to learn a whole lot of new words. You can increase it just by moving words from your passive vocabulary into your active one. Even better news: you don't have to add time to your schedule to do it! There are a couple of simple things you can do to start the process.
- Close the dictionary. This is the simplest and most important thing you can do. When you can't think of a word in your native language when talking with someone, do you stop and get out the dictionary? Not likely. My guess is that you try to find other words to express what you want to say. You may "talk around it" until you're understood. Once you come up with the word, you'll be much more likely to remember it next time. The same is true in a second language. If you can't find a word in your active vocabulary, talk around it. Often the other person will know what you mean and give you the word you're looking for. (That's what your English teachers are there for—let us do our job.)
- Stop using cheater words. We all have them: "good", "nice", "interesting", "thing/stuff", "really [+ cheater adjective/adverb]", etc. Rather than search for a more precise or expressive word, we cheat and use the easy one. What are your cheater words? Make a list, then stop yourself every time you start to use one and find a better word instead.
Doing just those two things will help you take unused words out of your passive vocabulary and plant them securely in your active one. Et voilà! Vocabulary expanded without any extra time spent.
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call up [phrasal verb]; to remember something
Et voilà [French expression] /ey vwala/—"and there you have it"; used to express the successful conclusion or delivery of something promised or predicted.