が公園で遊んでいた

was/were playing in the park

→ が

phrase Rare #42,164

was/were playing in the park The が marks the subject, and 遊んでいた means that the action was ongoing in the past. The subject can be singular or plural depending on context.

JA: 犬が公園で遊んでいた。

EN: A dog was playing in the park.

子供たちが公園で遊んでいた。

The children were playing in the park.

たくさんの子供達が公園で遊んでいた。

Many children were playing in the park.

子供たちの一団が公園で遊んでいた。

A group of children was playing in the park.