I love using previous student work to improve student writing. Sentence combining is the strategy of joining short sentences into longer, more complex sentences. The value of sentence combining is most evident as students recognize the effect of sentence variety (beginnings, lengths, complexities) in their own writing.
There are countless ways to gather student writing. Start class with a quickwrite. Have students return to a previous assignment. Ask them to write 5 sentences, either simple or compound. Once students have the baseline work, they can begin to work with complex sentences.
The following subordinating conjunctions are often used to connect related ideas: after, although, because, before, until, when. Subordinating conjunctions may come at the beginning of the sentence or in the middle of a sentence.