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The verb know is generally used with wh-clauses, so the noun clause must be what caused the fire. |
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The sentence reports a past event that has some present relevance, so it requires the perfect progressive aspect – have not been knowing. |
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The sentence lacks time expressions such as now, currently, and etc., so the progressive aspect cannot be used. |
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The verb know has a stative sense, so it cannot occur in the progressive aspect. |
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The sentence represents a usual habit, so it must be formed in the present simple tense. |