Learn Hardcore Korean: naega bonaen munjareul chinguga ilgeosseoyo. - My friend read the text message that I sent.

– 내가: marks “I” as the subject of the embedded action.
– 보낸: the past modifier form (verb stem 보내- + -ㄴ) of 보내다 “to send,” meaning “sent.”
– 문자를: 문자 “message/text” + object particle -를.
Altogether 내가 보낸 문자를 is a noun phrase “the message that I sent,” serving as the direct object of the main clause.

Why are there two subjects, 내가 and 친구가, in one sentence?

Korean marks the subject of each clause separately:
1) 내가 is the subject of the relative clause “(that) I sent.”
2) 친구가 is the subject of the main clause “(the friend) read.”
Even though both use the particle -가, they refer to different actions in different clauses.

How do I identify the direct object in this sentence?

The direct object is 문자 “message,” marked by the object particle -를, yielding 문자를. That tells you what 친구가 (the friend) acted upon when reading.

Why is the word order [내가 보낸 문자를] [친구가] [읽었어요] rather than subject–verb–object like in English?

Korean typically follows Subject–Object–Verb (SOV). Here you have:
• a relative clause (내가 보낸) modifying 문자,
• then the main-clause subject (친구가),
• and finally the verb (읽었어요).
Modifiers (like relative clauses) always precede the noun they describe, and the verb comes at the end.

What does 읽었어요 mean and how is it formed?

읽었어요 is the polite past tense of 읽다 “to read.” Formation:
읽- (stem) + 었- (past marker) + 어요 (polite ending) → 읽었어요, “(someone) read.”

Can I drop 내가 or 친구가 if the context is clear?

Yes. Korean allows subject/object omission (pro-drop) when it’s obvious who or what you’re talking about. For instance:
• 보낸 문자를 친구가 읽었어요 (drop 내가)
• 보낸 문자를 읽었어요 (drop both 내가 and 친구가, if context makes it clear)

Why is 내가 used instead of the more polite 제가, and can I substitute them?

나/내가 is informal “I,” while 저/제가 is the humble/polite form. Even in polite sentences (ending in -요), speakers often use 내가 in casual settings. To raise the politeness, switch to:
제가 보낸 문자를 친구가 읽었어요.