This is the 18216th most frequent Korean word.
술이었다
"술이었다" means "was alcohol" in English.
In this sentence, '술이었다' marks that the drink consumed during the mentioned event (어제 파티에서 마신 것) was alcohol.
어제 파티에서 마신 것이 술이었다.
What I drank at the party yesterday was alcohol.
The word '술이었다' is the past tense form of '술이다,' indicating that the subject (그것) was indeed alcohol in the past.
그것은 사실 술이었다.
That was actually alcohol.
Here, '술이었다' is used to emphasize that the beverage consumed by my friend (내 친구가 마신 것) was alcohol in the past.