You understand more than you can say? Native speakers do too! It's completely natural and there's no need to fight it. When learning a new word you first learn to understand it. As you hear the word more times it will surpass the threshold of speaking and you can use it.
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For real! I can read Dickens but can’t emulate his style spontaneously (although I could have a go at it after immersing myself in his prose for a while!)
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