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In an exercise I am to fill blanks with suitable prepositions
He does not leave his house ________ 9 o'clock.
Answer according to my book is before but I think I can also use till without any change in meaning.
Another sentence was
I must start ________ dawn to reach the station in time.
Answer according to book is at but dawn refers to a period of time how can one use at here. I think it should be before dawn
He does not leave his house until 9 o'clock.
This strongly implies that he does leave his house, and that he does so at "exactly" 9 o'clock (a reasonable amount of imprecision is allowed of course).
He does not leave his house before 9 o'clock.
This only weakly suggests that he does in fact leave his house. He may or may not. More importantly, it only asserts that if he does, he doesn't do it before 9 o'clock. He may do it at 9 o'clock or anytime after.
I must start at dawn to reach the station in time.
Your belief that dawn is a period of time rather than a specific moment is incorrect:
Dawn ... is the time that marks the beginning of twilight before sunrise. It is recognized by the appearance of indirect sunlight being scattered in the atmosphere, when the centre of the Sun's disc reaches 18° below the horizon.
Since it is a specific moment, at fits. (Here too, a reasonable amount of imprecision is assumed. It's unlikely the author will consult astronomical tables to calculate the exact time of dawn at that position on that day. Colloquially, "at dawn" means "whenever I notice the sun starting to come up.)
You may have been confusing dawn with twilight.
