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A compound sentence is usually TWO complete sentences that can be made into one sentence to make them flow better. There will, therefore, be more than one MAIN clause. To become a compound sentence, sentences should be joined together by a co-ordinating conjunction (such as for, and, nor, by, or, yet, so - note how they make FANBOYS- a good anagram to help you remember).
It is important to recognize that other conjunctions (though, when, etc) are subordinate conjunctions and they convert sentences to complex sentences NOT compound sentences.
The sentence "To my surprise, I found him in the hall" does not have two main clauses nor is it two complete sentences so we first have to change it to make it two complete thoughts; for example, 'to my surprise ' becomes 'I was surprised.' This is a complete sentence. 'I found him in the hall' is already a complete sentence. It then needs to be rearranged: 'I found him in the hall. I was surprised.'
"Conversational patterns" are important and the sentence has been rearranged because care must be taken when putting the two sentences together as it would make better grammatical sense to make this into a complex sentence, such as 'I was surprised when I found him in the hall.'
Although not a particularly interesting or descriptive sentence, the compound sentence then could be: "I found him in the hall and I was surprised."