Questions & Answers about Дети сидели на лужайке и ели яблоки.
Дети means children. It is the irregular plural of ребёнок (child).
Russian has a number of nouns with irregular plural forms, and this is one of the most common:
- ребёнок = child
- дети = children
So a learner just has to memorize this pair.
Because both verbs are in the past tense plural.
In Russian, past tense verbs agree with the subject in gender/number:
- masculine singular: сидел
- feminine singular: сидела
- neuter singular: сидело
- plural: сидели
Since дети is plural, both verbs take the plural past ending:
- сидели = were sitting / sat
- ели = were eating / ate
Unlike English, Russian does not need an extra word like were here.