Learn Hardcore Korean: chinguhanteseo siheom junbi gyojaereul billyeosseo. - I borrowed exam prep materials from a friend.

친구에게서 is the more formal equivalent and works the same way in polite speech.
친구한테 without 서 usually marks the indirect object (to a friend), not the origin. Adding 서 clarifies that the action comes “from” your friend.

Why is the 서 in 한테서 necessary? What nuance does it add?

The suffix -서 on case markers like 한테 or 에게 signals the starting point or source of an action.

한테 alone tends to indicate the target or recipient (to someone).
한테서 explicitly means “from someone,” so you know you got or borrowed something from that person.

Why is 시험 준비 placed before 교재, and how does this noun-noun modification work?

In Korean, modifiers always come before the noun they describe. Here, 시험 준비 (“exam preparation”) modifies 교재 (“materials” or “textbook”), forming the compound 시험 준비 교재 meaning materials for exam preparation. You stack the modifier directly in front of the head noun.

What is the role of 를 in 교재를 빌렸어?

를 is the object particle that marks 교재 as the direct object of the verb 빌리다 (to borrow). It tells us that 교재 is what was borrowed.

Why is there no explicit subject like 나는 or 저는 in this sentence?

Korean frequently omits the subject when it’s clear from context. In casual speech, if you’re talking about yourself and no ambiguity exists, you drop 나는/저는. The verb ending and situation imply that the speaker is the subject.

What does the ending -었어 in 빌렸어 indicate about tense and politeness?

The ending -었어 is an informal, low-politeness past-tense marker. It shows that the action (borrowing) is completed and is used among close friends or in casual settings.

Could you use 빌렸어요 or 빌렸습니다 instead of 빌렸어, and how would the meaning change?

Yes. Changing the ending adjusts the politeness level:

빌렸어요 is polite informal (for general polite conversation).
빌렸습니다 is formal polite (for very formal or official contexts).
The core meaning (I borrowed) stays the same; only the level of formality shifts.

In casual speech, can you drop 를 and just say 교재 빌렸어?

Informally, Koreans sometimes omit particles in fast or colloquial speech (so 교재 빌렸어 is understandable). However, especially when you’re learning, including 를 helps keep the sentence clear and grammatically complete.