Hi, "what do you do?" means "what is your job?" "How do you do?" is a polite way of asking "how are you?"

Hi Nanako, They do mean basically the same thing, but "hinge on" is a much stronger way of saying "depend on". For example, in the sentence you mentioned, "The success of the project hinges on the effectiveness of the ad campaign", the writer is implying that the ad campaign is the most important factor in the success of the project. If he had said "depend on" instead, the implication would be "the ad campaign is absolutely necessary to the success of the project." You could replace "hinge on" with "depend on", "hinge on" is just more emphatic. Hope this helps!