Why do you need understand independent and dependent clauses?

Independent clauses are part of sentences that can be written on their own and still make sense. Dependent clauses are parts of sentences that are not complete and so do not make sense. For example, an independent would be "He went to the store to buy milk". A dependent would sound like "since he was mad". You don't know what happens since he is mad, therefore making it a dependent clause.

i believe it’s the first one, but it could be the second! not the third or fourth.

explanation:

i decided to stay late in the laboratory to continue my experiment with the lightbulb. at midnight, thomas edison opened the door and saw all the stuff he was working on. the lightbulb, film and the transmitter. as he walks in an idea hits him on how the lightbulb would work. he would have to change a few wires around to get it to light up. when he did change the wires around it worked so he decided to write a telegraph to his friend to tell him about his success.   as he was packing up he grabbed the telegraph and walked to his home. he told his wife about the success and the next morning mailed the telegraph.