A dash is commonly used to indicate a pause, they are also used to separate groups of words, we can use them as a substitute for parenthesis at the end of sentences, or to add extra or detailed information.
The sentence that shows the correct use of the dash is I am so mad at you for spilling soda on my homework that I could—oh, forget it.
The last part of the sentence expresses a consequence and it adds something extra to the idea as well.