Read through newspapers and books to find extended sentence formations. Use your highlighter to identify them, so you can understand how to write them yourself. An extended sentence is one phrase, with a second idea or notion attached. Key words to indicate this include, "and," "but," "because," "since," or "although."
Compile a list of the words used to extend the sentences you have highlighted. You can refer to this list when writing your own sentence. If you are unsure if a sentence has been extended try to divide it into two sentences, separated by a period. This could indicate that it contains more than one phrase, or idea, and in fact is an extended form of a sentence.
Build your own sentence. First, write down a basic phrase that includes the subject, verb and object; such as "I dropped the ball."
Continue the sentence you wrote during step three, using the word "because" to practice how to form an extended sentence. This is a really common word that will help you learn to combine two ideas in one. For example, write, "I dropped the ball because I was not looking in the right direction."
Extend sentences using more than one joining word. For example, write, "they managed to get there, although the traffic was slow and it was wet." In this example, there are two clauses. The first, "they managed..." then the second beginning "although." Each of these phrases is an independent idea. Also, note that the second clause contains two joining words, "although" and "and," to create a longer sentence.