Learn Hardcore Korean: sijangeseo chingureul mannayo.

The particle -에서 marks the location where an action takes place. Here, the meeting happens at the market. If you used 시장에, that would only mark the destination (“to the market”) or a location of existence (like “is at the market”) but not specifically the site of an action.

What does the particle -를 do in 친구를?

-를 is the direct‐object marker. It tells you that 친구 is the object of the verb 만나요 (i.e., the thing being met).

What is the tense and politeness level of 만나요, and how is it formed?

만나요 is present tense, polite-informal speech (해요체). It’s built from the verb stem 만나- plus the ending -아요. Because the stem ends in the vowel ㅏ, you attach -아요, which contracts naturally into 만나요.

Why is there no subject in this sentence? Who is doing the meeting?

In Korean, subjects are often omitted when they’re clear from context. Here, the implied subject is usually I (저) or we (우리), depending on who’s speaking.

Why does the verb come at the end of the sentence? Can I change the word order?

Korean follows an S-O-V (Subject-Object-Verb) order, so the verb goes last. Although you can rearrange elements for emphasis (using topic or contrast markers), the verb almost always stays at the end.

How do you say this sentence in the past tense?

Change -아요 to -았어요 (because the stem vowel is ㅏ).
시장에서 친구를 만났어요.
(“I met a friend at the market.”)

How do you say this sentence in the future tense?

Use the future construction -을 거예요 (or more casually -ㄹ 거야).
시장에서 친구를 만날 거예요.
(“I will meet a friend at the market.”)

How can I turn this into a question, like “Do you meet a friend at the market?”

Keep the same ending and simply use a rising intonation or add a question mark:
시장에서 친구를 만나요?
If you want a more honorific question, you could say:
시장에서 친구를 만나세요?