jeonghwa hwaldong dwieneun sanchaekroga kkaekkeushaejyeossseupnida.

정화 활동 is a noun phrase meaning a cleanup activity or cleaning campaign.

• 정화 = purification, cleanup, making something clean
• 활동 = activity

In this kind of context, 정화 활동 usually refers to an environmental cleanup effort, such as picking up trash in a park, along a trail, or near a river.

뒤에 means after.

So:

• 정화 활동 뒤에 = after the cleanup activity

The extra 는 makes that time phrase a topic or adds a slight contrast/emphasis:

• 정화 활동 뒤에는 = as for after the cleanup activity... / after the cleanup activity, ...

This often feels natural in Korean when setting the scene or highlighting a time point. The sentence would still be grammatical with 뒤에, but 뒤에는 sounds a bit more like the speaker is framing that time period specifically.

Because 깨끗해졌습니다 means became clean, and the thing that changes state is marked as the subject with 가.

So:

• 산책로가 깨끗해졌습니다 = the walking trail became clean

If you used 를, it would suggest the trail is a direct object of an action, but here the trail is not being directly acted upon by the verb become. It is the thing that underwent the change.

깨끗해졌습니다 comes from:

• 깨끗하다 = to be clean
• -아/어지다 = to become
• -습니다 = formal polite sentence ending

So:

• 깨끗하다 → 깨끗해지다 = to become clean
• 깨끗해졌습니다 = became clean / has become clean

This form focuses on a change of state, not just the condition itself.

Because the sentence is talking about a resulting change.

• 깨끗했습니다 = was clean
• 깨끗해졌습니다 = became clean

Since the trail was cleaned up after the activity, 깨끗해졌습니다 is the natural choice. It shows that the trail was not clean before, and then it became clean.

Yes, Korean often leaves out the doer when it is obvious from context or not important.

This sentence does not explicitly say who cleaned the trail. It only says that:

• after the cleanup activity
• the trail became clean

The people who did the cleanup are understood from context, so Korean does not need to mention them.

Korean commonly puts time expressions early in the sentence.

So the structure is:

• 정화 활동 뒤에는 = after the cleanup activity
• 산책로가 = the walking trail
• 깨끗해졌습니다 = became clean

This is very natural Korean word order: time/topic + subject + predicate.

English usually says something similar too: After the cleanup activity, the trail became clean.

산책로 means a walking path, trail, or promenade.

It is made up of:

• 산책 = walk, stroll
• 로 = road/path

So 산책로 is a path meant for walking or strolling, such as one in a park or nature area.

It is in the formal polite style, often used in:

• presentations
• news reports
• public writing
• speeches
• formal conversation

Some less formal versions would be:

• 깨끗해졌어요 = polite everyday style
• 깨끗해졌어 = casual style

So the sentence sounds polite and somewhat formal.

Yes. 뒤에 and 후에 can both mean after.

• 정화 활동 뒤에는
• 정화 활동 후에는

Both work, but there is a slight difference in feel:

• 뒤에 often sounds a bit more common and conversational
• 후에 often sounds a bit more formal or written

In this sentence, both are natural.