Passive vs. active vocabulary

Even if we learn a word, it takes a lot of practice and context connections for us to learn it well. A rough grouping of words we understand when we hear them encompasses our "passive" vocabulary, whereas our "active" vocabulary is made up of words that come to our mind immediately when we have to use them in a sentence, as we speak. In this case, we often have to come up with a word in the timeframe of milliseconds, so one has to know it well, often in combinations with other words in phrases, where it is commonly used.