ごちそうさま

How it works

ごちそうさまでした (thanks for the meal) is a set phrase said after eating — its でした (was) is the past polite ending, so it always comes out in this fixed shape. Then comes the verdict: おいしい (delicious) is an i-adjective, and to put it in the past you drop the final い and add かった, giving おいしかった (was delicious). There's no subject — the meal is understood — so the bare adjective stands alone, sounding warm and a little informal after the polite opener.