com/definition/us/when : After which; and just then (implying suddenness) ‘he had just drifted off to sleep when the phone rang’ However, the punctuation of the sentence does not encourage this interpretation. Regardless of any technical rules, I think the sentence is easier to read like this: The army begins to march, when the tide suddenly pulls away, leaving dry land where there was ocean before.
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AnonymousThe army begins to march when the tide suddenly pulls away, leaving dry land where there was ocean before.My guess is that your teacher sees "army" as the implicit subject of "leaving dry land" when the comma is present, which is the wrong interpretation. I would leave out the comma to prevent that confusion.