Start with a simple sentence. All sentences must fully express an idea or thought. A simple sentence includes a subject --- the who or what that is doing something --- and a verb, or an action. Take the following example: 'Alan ran.' 'Alan' is the subject, and 'ran' is the verb.
Add an adverb to the simple sentence to increase the level of descriptive detail. Adverbs are generally added after the verb or between the noun and the verb. For example, 'Alan ran quickly', or 'Alan quickly ran.' In this sentence, 'quickly' is the adverb.
Build sentences by adding conjunctions which allow you to join two simple sentences together. Use common coordinating conjunctions --- and, but or or --- to create compound sentences. Compound sentences are created when two simple sentences are combined. For example, 'Alan ran, and Max swam.' This is a sentence in its own right, but both parts of this complex sentence can stand alone as simple sentences. A comma must come before the coordinating conjunction.
Use subordinating conjunctions to form complex sentences. A complex sentence is made up of an independent clause, or simple sentence, and a dependent clause. Dependent clauses cannot stand on their own and begin with a subordinating conjunction. Some common subordinating conjunctions are because, since, when, if, while and after. Depending on the conjunction used and the meaning intended, the conjunction should be placed either between the two parts of the newly-created sentence, or at the start of the newly-created sentence. Continuing with our example, we can create the sentence 'While Alan runs, Max swims.' The dependent clause is 'While Alan runs.' The independent clause is 'Max swims'. When the sentence begins with a dependent clause, a comma must follow it. An equally valid complex sentence, using the same words with the conjunction in a different position to create a different meaning, is 'Alan runs while Max swims.'