What are the 5 most important table manners?

Short answer

The five most important table manners are: wait until everyone is served before eating, chew with your mouth closed, use your cutlery from the outside in, say please and thank you, and avoid reaching across the table.

The five most important table manners are simple and easy to remember.

Wait until everyone at your table has been served before you start eating. If the host or the couple begin first, follow their lead.
Chew with your mouth closed and avoid talking with food in your mouth. Take small bites so you can join the conversation easily.
Use your cutlery from the outside in. At a formal wedding, the starter fork is on the outside, and you work towards your plate with each course.
Say please and thank you to the waiting staff. They are working hard to make the meal pleasant, and courtesy costs nothing.
Avoid reaching across the table. If you need something, ask the person nearest to it to pass it to you.

These manners apply whether you are at a plated dinner, a buffet, or a relaxed barbecue. They show respect for your fellow guests and for the people serving you. If you are unsure which fork to use, watch the person hosting your table or the couple. When in doubt, start from the outside and work inwards. Good table manners are not about being perfect. They are about making the meal comfortable and enjoyable for everyone.