The knife in dreams is used as the tool to hurt the others. That is why it is almost never a good omen. You feel you need to protect yourself or you wish to do the harm to others. The penknife indicates the legal troubles the dreamer will face, but only it is opened, it is closed, he will do well. The rusty knife signifies marriage problems and the sharp one very personal ones. The knife that is not sharp and is dirty, indicates the hard work the dreamer will have.
Read more about dreaming of Knife in other dream meanings interpretations.