Can I start a sentence with Then? [closed]

Yes, you can start a sentence with then. However, the clause that begins with then should go last:

She asked, "Could I have a drink of your water?" I said, "No, you can't." Then she walked away.

Then indicates a consequence or a result, which is why it should go at the end of any group of sentences that describe actions that create that consequence.

Think of then as a result of if, where if always comes first:

If she may not have a drink of your water, then she will walk away.

He read the letter and began to cry. His huge old saltwater aquarium burst.Then the room was flooded with saltwater.

or:

He read the letter and began to cry. Then the room was flooded with saltwater.

His huge old saltwater aquarium had burst.

There is a rhyme and reason to where and when "then" appears. The second example is clearly superior because "then" is placed "incorrectly". In the second example the placement of "then" gives us the impression that the room was flooded with tears, before we are told the more mundane cause. In the first example, the flooding clearly follows, and is a consequence of, the bursting aquarium. This is much drier.

My point is that grammar is meant to serve the story, to make things more clear (or deliberately ambiguous), to create tension and release. If you keep your eyes on the prize, so to speak, you will make fewer errors without having to memorize rules, and the errors you do make will be different in kind than the errors of sloppy writing.