Use the adverb hitherto when you're describing a state or circumstance that existed up until now. If you find a hitherto undiscovered hidden tunnel in your cellar, you're the first to discover it.
When some particular situation has existed so far, up until the moment that you're speaking of, you can use the word hitherto to talk about it. You could say that you're taking a hot air balloon ride that you were hitherto too scared to contemplate, or that a very shy girl in your philosophy class who was hitherto totally silent finally spoke up today and asked a very intelligent question.
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