At Dinner Table
잘 먹겠습니다 & 잘 먹었습니다.
The situation is that You are invited to or visiting Korean family. Say you went to your Korean friend’s house (just visiting). It was about a lunch time and your friend’s mom somehow prepared lunch for you guys. So, you and your friend sat down on the table and your friend’s mom is around. Probably she will say “help yourself” in Korean. In this situation, right before you pick up the spoon or chopsticks, you can say,
“잘 먹겠습니다.” / Jal meokgetseumnida/,
meaning “I will eat very well,” and this “잘 먹겠습니다.” also implies “Thank you for preparing food.”
Now, you have just finished lunch and you are very happy. At this point your friend’s mom is also around to clean up the dishes and this time you can say,
“잘 먹었습니다.” / Jal meogeotseumnida/
meaning (literal) “I ate very well,” and this also implies “It was really good. Thank you so much for the food.”
In general you will say “잘 먹겠습니다” and “잘 먹었습니다” to the people who cook or buy meals for you. However, you can say to yourself too. For example, you went to grocery and prepared meal by yourself. Even in this situation you can say “잘 먹겠습니다” and “잘 먹었습니다” to yourself or to the higher power. These simple phrases also can be used in the restaurant even though you are paying for food. Say, waiter or waitress brought all dishes to you and you are about to eat and you can still say “잘 먹겠습니다” to your waiter or waitress. In this case, you are thanking restaurant’s effort of cooking your meal. So TRY "잘 먹겠습니다.” when you have a dinner tonight.