Just Looking ...

As Prof. Karmar suggested, we can't change the present by changing the past. This is illustrated well in the 2002 remake of "The Time Machine" based on a novel by H.G Wells.

Near the begining of the movie, the protagonist's fiancé is killed, and in order to save her life he invents a time machince to travel back in time to prevent her death. Although he succeeds saving her from being shot, to his dismay she is killed yet again a mere few hours later. No matter how many times he tries to alter the past, the ultimate consequence is of his fiancé dying (even though the cause of death is different).

I just thought this as a good example to what the professor was talking about. Although, I am curious as to how nothing else changed in the present when the past was altered, because we are lead to belive that if one thing changes in the past, the present might be completely different. In this plot, nothing else changes except for the way she dies.
This also leads me to another question. The time machine was invented because of the fiancé's death, and if she had lived it would have never existed. So how is it possible for the protagonist to use the time machine to go back in time to save her, and then use it right after to return to his present? The minute he saves her in the past, the time machine should have become non-existent, restricting him from travelling back to his original time. This then leads to the matter of there being 2 clones in the past (since he would've been stuck in that time period). If anyone has seen this movie, and has an explanation or if I'm wrong about the plot or sequence of events, I would love to hear it.