Alternate structures of clauses.

0Hello everybody.02br
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00In Grammar, adverbial clauses have alternate structures, this means changing the structure of the clause into a new one, it can be either: coordinate clause, prepositional phrase with gerund or with abstract noun, participal phrase or an absolute constraction.02br
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00We have the following exemple:02br
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00After we finished breakfast, we went for a walk.02br
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00Coordinate Clause: We finished breakfast; then we went for a walk.02br
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00Prepositional phrase with Gerund: After finishing breakfast, we went for a walk.02br
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00Participal Phrase: Having finished breakfast, we went for a walk.02br
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00Absolute Construction: Breakfast finished, we went for a walk.02br
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00My Qs are:02br
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00a/"Then" is not among the coordinate conj. we studied (for, and, nor, but, or, yet, so). Why is it used in the Coordinate clause?02br
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00b/ What are the appropriate prepositions we can use to refer to time, place, concession, purpose, reason and result? 02br
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00Thank you!0-