was - 었 That's the easist way to explain and catch the concept of Korean past tence. 나는 사과를 먹는다. - I'm eating an apple. 나는 사과를 먹었다. - I ate an apple. 그는 집에 있다. - He is at home. 그는 집에 있었다. - He was at home So if you want to make a paste tence, then you can just put 었 right next to verb's root. Ex) 있다's root is 있. so 있다's past tence will be 있었다. 먹다's root is 먹. so 먹다's past tence will be 먹었다. I hope my explanation will be helpful to you :)

그녀가 콘서트에 있었어요 She was at the concert ~에 있다 to be at 있었어요 - past tense

Just remember, in English, we use the verb "to be" to mean "equality" (My neighbor is a teacher.) and "existence" (She was at the concert.). In Korean, they have two different words for that: 이다 for equality and 있다 for existence.