Like all independent clauses, the main clause must contain a tensed verb and a subject. In complex sentences, the independent clause is the main clause. Main clauses do not have a fixed place in the order of complex sentences--they may begin the sentence, come in the middle, or end the sentence.
Since the sun and Earth are embedded in the galaxy, it is
difficult for us to obtain an overall view of the galaxy.
--"The Milky Way," Compton's Encyclopedia
The main clause should contain the most important information in the sentence.
For more on main clauses and ordering, see Placement of Phrases and Clauses.