Learn Hardcore Korean: madange aideuri ttwieonolgo isseo. - Children are running around in the yard.

뛰어놀다 is a compound verb made from 뛰다 (“to run”) + 놀다 (“to play”), so together it means “to run around and play.” In Korean it’s treated as one verb, not two separate actions.

Why is it 뛰어놀고 있어 and not 뛰고 놀고 있어?

Because 뛰어놀다 is a single verb. To make it progressive, you attach –고 있다 to its stem:
• 뛰어놀 + 고 있다 → 뛰어놀고 있다
In casual speech –고 있다 contracts to –고 있어. Writing 뛰고 놀고 있어 would split it into two verbs (뛰다 and 놀다) and change the nuance.

How do you form the present progressive –고 있어 in Korean?

Take the dictionary form of the verb and drop –다 to get the stem.
Add –고 있다 for the continuous aspect.
In spoken or informal writing, 있다 often shortens to 있어.
Example with 먹다 (“to eat”):
• 먹다 → 먹 + 고 있다 → 먹고 있다 → 먹고 있어 (“is eating”)

What is the function of 들 and 이 in 아이들이?

• 들 is a plural marker. 아이들 means “children/kids.”
• 이 (subject marker) attaches to 아이들 to show it’s the subject of the sentence.
Together, 아이들이 means “the kids (are …).”

What does 마당에 mean, and why is the location marked with –에 instead of –에서?

• 마당 means “yard,” and the particle –에 marks the location (“in/at the yard”).
• Normally, –에서 is used to indicate where an action takes place (e.g., 마당에서 뛰어놀고 있어). However, in everyday spoken Korean you’ll often hear 마당에 for simplicity, without changing the overall meaning.

Why is the verb 뛰어놀고 있어 placed at the end of the sentence?

Korean uses a Subject–Object–Verb (SOV) word order. Adverbial phrases (like 마당에) and the subject (here 아이들이) come before the verb, which always appears last.