Questions & Answers about Дело в том, что я не знал вашего адреса.
Why do we say в том, что here instead of just что?
This is a classic example of a pointer word construction. In English, we just say "The thing is that" but in Russian, the noun дело connects to the preposition в (literally: "The matter is in"). Because a preposition cannot attach directly to the conjunction что, Russian uses том (the prepositional case of то) as a bridge. Literally, it means "The matter is in that, that".
Is дело в том, что a fixed phrase, or can I change the words?
It is a highly common, fixed conversational phrase. You will hear it constantly used to mean "The thing is that," "The fact is that," or even "The problem is that." It is best memorized as a set expression exactly as it is.
Why is адрес in the genitive case as вашего адреса instead of the accusative ваш адрес?
This happens because of the negated verb не знал (did not know). In Russian, when a transitive verb (a verb that takes a direct object) is negated, its object often switches from the accusative case to the genitive case. This "genitive of negation" is very common with verbs of perception or knowledge like знать.
Does the form знал indicate anything about the speaker?
Yes, знал is the masculine past tense form, which tells us the speaker is male. If a woman were saying this sentence, she would use the feminine form and say я не знала.