Learn Hardcore Korean: jinan jue hakgyo geonmul apeseo chingureul mannasseoyo. - I met a friend in front of the school building last week.

학교 건물 is a compound meaning “school building” (학교 modifies 건물).
앞 means “front.”
-에서 indicates where an action takes place.
Putting them together in Korean order—[compound noun] + [place noun] + [location particle]—gives 학교 건물 앞에서: “in front of the school building.”

What’s the difference between the particles -에서 and -에?

-에 marks a static location (“at”), a destination (“to”), or a time point (“on/at”).
-에서 marks the location where an action occurs (“at” in the sense of “at the park, I ran”).
Because 만났어요 (“met”) is an action, we use -에서 for the place.

Why is 친구를 marked with -를?

-를 (after a vowel) or -을 (after a consonant) is the direct-object marker in Korean. The verb 만나다 (“to meet”) takes a person as its object, so 친구 (“friend”) becomes 친구를.

How is 만났어요 formed, and what nuance does it carry?

Stem: 만나- (from 만나다).
Past tense marker: -았 (만나 + 았 = 만났).
Polite ending: -어요.
Altogether, 만났어요 is the polite past-tense form meaning “met.”

Why isn’t there a subject in the sentence? How do we know who met the friend?

Korean often omits subjects when they are understood from context. Here, it’s clear you (저/나) are the one who met a friend, so 저는 or 나는 is dropped.

Can 지난 주에 be written without a space, like 지난주에?

Yes. 지난주 is a commonly accepted compound for “last week,” so you’ll see both 지난 주에 and 지난주에. Modern usage often favors the one-word form, but both are correct.

How would you turn this sentence into a question—“Did you meet your friend in front of the school building last week?”

You can simply raise the intonation or add a question mark:
지난 주에 학교 건물 앞에서 친구를 만났어요?
For a slightly softer or more formal inquire, use -나요 or honorifics:
지난 주에 학교 건물 앞에서 친구를 만났나요?
지난 주에 학교 건물 앞에서 친구를 만나셨나요?