Both sentences are possible. These come to my mind: I didn't know (before you told me) why he is leaving (tomorrow). I didn't know (last week) why he was leaving (last week).
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Ahmed AlsmairBut I have a question shouldn't the first sentence be phrased like :I didn't know why he is going to leave tomorrowI didn't know why he will be leaving tomorrowOrI didn't know why he will leave tomorrowNo. It's correct English as it is. Of course more or less the same idea could be expressed in other wor