There are always several ways to combine a sentence. First of all, there are three basic types of sentences: simple, compound, and complex. If you combine sentences, you will usually have compound or complex sentences unless you use very few parts of the sentences. For example, you can use adjectives and adverbs and prepositional phrases.
Six whistling boys came running hard over the hill with their heads down and umping.
In this case, there is a simple sentence. You can also use a compound sentence by combining the sentence with two separate independent clauses.
Six boys were running over the hill with whistling breath and their heads were down and umping.
That is two independent clauses joined by the conjunction "and." Notice that there are no conjunctions in the other sentence except in the phrase.
Finally, you can make a sentence complex or compound complex by using dependent (subordinate) clauses.
Six boys came running hard down the hill while their heads were down and umping.
This sentence has a subordinate clause because of the word "while" used to join the sentences.