Hard start then engine shuts off when transmission is shifted into gear

I have been having issues with my car for several weeks now. Two mechanics have looked at it and aren't sure of the problem. At first, I just had problems getting it to start so after looking it over a couple times, the first guy said it needs a fuel pump. Okay $200.00 down. Two days later, same issues. He said it now needs plugs and wires because fuel is getting in so it won't start. Okay, more money. Still having issues and after 3 weeks, he still can't figure it out. I finally had to get a battery just because. Now it starts, hesitatingly, and idles fine but shuts off when put in gear. I smell gas, it smokes a little, backfires, sputters a lot. What would you guess is the problem?