Learn Hardcore Korean: yaksok siganeul mesijiro allyeo juseyo. - Please let me know the appointment time via message.

알리다 = to inform/notify → connective -아/어 → 알리어 → 알려
주다 as an auxiliary = “do it for someone” → 알려 주다 = inform (for me)
Polite request ending -세요 → 알려 주세요 = please inform (me) This is the natural, polite way to ask someone to provide information.

Why is it 약속 시간을 with -을? Can I drop the particle?

-을/를 marks the direct object. 약속 시간 (“appointment time”) is what you want to be told, so 약속 시간을 is standard.

In casual speech, you might hear the particle dropped: 약속 시간 메시지로 알려 주세요. It’s understandable but less careful/formal. Keep -을/를 in writing or neutral/formal speech.

What does 메시지로 mean, and why -로 not -으로?

-로/으로 marks the means or method: 메시지로 = “by/through a message.” Choice rule:

After a vowel or the consonant ㄹ → -로 (메시지 ends in a vowel → 메시지로)
After other consonants → -으로 (e.g., 문자 → 문자로; 카카오톡 → 카카오톡으로) Note: 메시지에/에서 would mean “in/at a message,” not “by message,” so they don’t fit here.

Can I say 문자로 instead of 메시지로? What about apps and email?

Yes. Common options:

문자로 = by text/SMS
카톡으로/카카오톡으로 = via KakaoTalk
이메일로 = by email
전화로 = by phone call Examples: 약속 시간을 문자로 알려 주세요. / 카톡으로 알려 주세요.

Is the word order fixed? Could I say 메시지로 약속 시간을 알려 주세요?

Word order is flexible. Both are natural:

약속 시간을 메시지로 알려 주세요.
메시지로 약속 시간을 알려 주세요. Putting 약속 시간을 first lightly emphasizes the item; putting 메시지로 first highlights the method.

Where is “me” in this sentence? Should I add 저한테/저에게?

Korean often omits obvious participants. Here, the recipient “me” is understood.

Adding it is fine if you want to be explicit: 저한테(=to me) 메시지로 알려 주세요 or 저에게 메시지로 알려 주세요.
한테 is more conversational; 에게 is more formal/written.

Is it 알려 주세요 or 알려주세요? Which spacing is correct?

Both are widely seen, but standard orthography prefers spacing the auxiliary verb: 알려 주세요 (main verb + auxiliary 주다). In careful writing, use the space. In casual texting, 알려주세요 is common and acceptable.

Why is there a space in 약속 시간? I often see 약속시간.

In standard spacing, a modifier noun plus head noun is written with a space: 약속 시간 (“appointment time”). Closed-up 약속시간 appears informally, but formal writing favors the spaced form.

Difference among 알려 주세요, 말해 주세요, and 가르쳐 주세요?

알려 주세요: “please let me know/inform me”—neutral and very common for facts like times, addresses, etc.
말해 주세요: “please tell me”—focus on saying it; also fine here.
가르쳐 주세요: literally “please teach me,” but in colloquial Korean it often means “tell me (the info).” Slightly more old-fashioned or learnerish in some contexts but still common.

Why 알려 and not 알아? What’s the difference between 알리다 and 알다?

알리다 = to inform/let someone know → 알려 주세요 asks the other person to provide information.
알다 = to know/understand → 알아 주세요 means “please understand/please be aware,” which is a different request.

Can I use the topic marker: 약속 시간은 메시지로 알려 주세요?

Yes. 약속 시간은 makes “the appointment time” the topic, often with a contrastive feel: “As for the appointment time, please let me know by message (as opposed to something else).” Use -은/는 when contrasting or setting a topic.

How can I adjust politeness or formality?

Casual: 약속 시간 문자로 알려 줘.
Polite neutral: 약속 시간을 메시지로 알려 주세요.
Softer/indirect: 약속 시간을 메시지로 알려 주시겠어요? / 알려 주실 수 있나요?
Formal/business: 약속 시간을 메시지로 알려 주시기 바랍니다. / 알려 주십시오. Note the spelling: 주십시오 (not “주십시요”).

What’s the nuance difference between 알려 주세요 and 보내 주세요 here?

알려 주세요 focuses on providing the information (any method, but you’ve specified “by message”).
보내 주세요 focuses on sending something. With this content, you can say 약속 시간을 메시지로 보내 주세요 (“send me the appointment time by message”), which is also natural.

Should I end this with a period or a question mark?

Grammatically it’s an imperative request, so a period is standard: …알려 주세요. In casual messages, a question mark is often used to soften the tone: …알려 주세요? Both are common in texting; in formal writing, use a period.

When do I use 드리다 instead of 주다, like 알려 드릴게요?

Use 드리다 when the subject is you (or your in-group) doing something for a higher-status recipient—it’s the humble form of 주다.

Asking someone else: 알려 주세요/주십시오 (you want them to do it).
Telling someone you will do it: 제가 약속 시간을 문자로 알려 드릴게요 (“I’ll let you know the time by text”).

Is 시간 the best word? What about 시각 or other options?

시간 can mean “time” (both point in time and duration); in everyday speech it’s used for specific times too.
시각 means a specific clock time and is more formal/literary. You could say 약속 시각을…, but it’s uncommon in casual speech.
If you want the date: 약속 날짜를…; the place: 약속 장소를…; all details: 약속 내용을… or 약속 관련 정보를….

Could I just say 약속을 메시지로 알려 주세요?

You can, but it changes the scope:

약속 시간을… asks specifically for the time.
약속을… sounds like “let me know the appointment (details)”—broader, possibly including time, place, etc.

Any common mistakes to avoid with this sentence?

Using the wrong particle for means: avoid 메시지에/으로서 here; use 메시지로.
Mixing up verbs: avoid 알아 주세요 when you mean “let me know.”
Misspelling the formal request: 주십시오 (not “주십시요”).
Over-marking recipients with honorific to self: don’t say 저께; use 저에게/저한테 if needed.