chingudeuri gongwoneseo norayo.

이/가 are both subject markers, but you choose based on the final sound of the noun: • Nouns ending in a consonant take 이 (친구들 → 친구들이).
• Nouns ending in a vowel take 가 (학교 → 학교가).
They both mark the noun as the subject of the verb.

• 은/는 is the topic marker: it introduces or contrasts the topic of conversation.
• 이/가 is the subject marker: it indicates who or what is performing the action or existing in a state.
In 친구들이 공원에서 놀아요, using 이/가 highlights who is playing (i.e. the friends) rather than making friends the general topic.

-에서 is the location particle used with action verbs.
• 공원 means “park.”
• 공원에서 놀아요 literally means “(they) play at/in the park.”
-에서 tells you where the action takes place.

-아요 is the polite present-tense verb ending for stems with the vowel ㅏ or ㅗ.
Verb stem: 놀- (from 놀다, “to play”)
Conjugation: 놀 + 아요 = 놀아요
This makes the sentence polite and present tense.

Use -았어요 for stems with ㅏ or ㅗ: • Stem: 놀-
• Past tense: 놀 + 았어요 = 놀았어요
So it becomes 친구들이 공원에서 놀았어요. (“My friends played at the park.”)

You can simply raise your intonation at the end in spoken Korean: • 친구들이 공원에서 놀아요?
Or use a more formal question ending -습니까?: • 친구들이 공원에서 놉니까?
Both forms ask “Are (my) friends playing at the park?”