car wont start

We own a 1998 Nissan Sentra. A week ago we got up one morning and the car wouldn't start. We explained what it was doing to a mechanic friend and he suggested replacing the fuel pump, but to try the filter first to be sure. We did this and the car ran fine for 3 days. We got up that morning and the car wouldn't start again. We finally got it to start, but the gas peddle wouldn't rev the engine. Whenever you push it it just clicks. The car is sputtering and knocking when it is on, what could be the problem or the best solution?

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