Learn Hardcore Japanese: mikkainai ni watasi ni renrakusite kudasai.

Reading: みっかいないにわたしにれんらくしてください (romaji: Mikka inai ni watashi ni renraku shite kudasai). In normal Japanese, spaces are not used: 三日以内に私に連絡してください.

What does 以内に mean, and how is it different from までに?

以内に = “within (not exceeding), including the limit.” 三日以内に means “within three days,” including the third day as part of the allowable window.
までに = “by (no later than),” also inclusive. It’s used with a specific deadline point: 金曜日までに (by Friday).
With a number of days, both can create a deadline, but natives tend to prefer:
- 数値系の範囲: 三日以内に (within 3 days).
- 特定時点: 金曜日までに (by Friday). Be careful: 三日までに is often read as “by the 3rd (of the month),” not “within three days.”

Does 三日 (みっか) here mean “three days” or “the 3rd (of the month)”?

Here it means “a duration of three days.” The presence of 以内 forces the “amount of time” reading. If you meant a date, you’d normally say 三日に (“on the 3rd”) or 三日までに (“by the 3rd [of the month]”).

Why are there two に particles: 三日以内に私に? Is that okay?

Yes. They mark different things:

First に after 三日以内 marks the time limit (deadline).
Second に after 私 marks the indirect object/recipient (“to me”). Multiple particles can appear as long as they mark different roles.

Can I drop 私? Or say 私へ instead of 私に?

Dropping 私 is very common when it’s obvious who should be contacted: 三日以内にご連絡ください.
私に vs 私へ: both are acceptable with 連絡する. に is more common in speech; へ feels a bit more written/formal.
You may also see nuance markers:
- 私までご連絡ください (to me, not elsewhere)
- 私宛(あて)にご連絡ください (“address it to me,” used for mail/email destinations)

What is the role of 連絡して and ください here?

連絡する is a suru-verb meaning “to contact.” Its て-form 連絡して links to a request.
ください makes a polite request: “please do X.”
Together, 連絡してください / 連絡してください means “please contact (me).” In normal writing there’s no space.

Is ご連絡ください better than 連絡してください?

ご連絡ください adds the honorific prefix ご to the Sino-Japanese noun 連絡, making it more polite and businesslike. It’s the default in formal emails and notices.
Neutral: 連絡してください
More formal: ご連絡ください
Very formal/softer: ご連絡いただけますか, ご連絡のほどよろしくお願いいたします

Can I say 連絡ください without して?

Yes. 連絡ください is a common, slightly clipped request (polite but casual-business). In more formal contexts, prefer ご連絡ください.

Is ください written in kanji (下さい)?

When used as the request auxiliary (“please do …”), style guides recommend kana: ください. The kanji 下さい is typically reserved for the main verb “please give me (something).” You’ll see both in the wild, but ください (kana) is safest here.

Is it okay to say 三日間以内に?

Avoid it. Natives overwhelmingly say 三日以内に. Adding 間 is unnecessary and sounds awkward in most contexts.

Does 以内 include the boundary? Is the third day included?

Yes. 以内 is inclusive. 三日以内に allows up to and including the third day (relative to the reference time).

From when do we start counting the “three days”? Does “today” count?

It’s from the relevant reference time, which is usually:

The moment of speaking/writing (“within three days from now”), or
A specified point mentioned earlier (e.g., “within three days of delivery”). Whether “today” counts depends on what “three days” means operationally. If you need absolute clarity, specify it:
本日から三日以内にご連絡ください (within three days starting today)
受領日から三日以内にご連絡ください (within three days from the receipt date) For exact hours, use hours: 72時間以内に.

Can I use で instead of に after 以内?

For deadlines attached to actions, use 以内に. 以内で appears in other contexts (e.g., choices/limits: 5000円以内で買える), but for “do X within Y (time),” say Y以内に V.

What objects can 連絡する take? Why not を after 私?

Recipient: marked by に/へ (e.g., 私に連絡する).
Content/matter: can be a direct object with を (e.g., 日程を連絡する, “to inform the schedule”) or expressed with について (“about”).
You can nominalize: 私に連絡をする/ください is also fine, but don’t mark the recipient with を.

How flexible is the word order?

Japanese word order is flexible. These are all acceptable:

三日以内に私に連絡してください (most natural)
私に三日以内に連絡してください (also fine) You generally keep the verb at the end and place time expressions early. Don’t split 連絡してください.

What’s the difference between 三日以内に and 三日後に?

三日以内に = “sometime within the next three days” (deadline window).
三日後に = “in three days (exactly at the time three days later).” Different ideas: window vs exact point.

Is 三日中に a good alternative?

No. 〜中に means “sometime during [that period/day].” 三日中に would suggest “sometime on the 3rd (day of month),” not “within three days.” Use 三日以内に for “within three days.”

How do I make this much more formal or softer?

Common business: 三日以内にご連絡ください。
Softer request: 三日以内にご連絡いただけますか。
Very formal/formulaic: 三日以内にご連絡のほどよろしくお願いいたします。
With “at the latest”: 遅くとも三日以内にご連絡ください。

What if I want to say “by Friday” instead of “within three days”?

Use までに with a named deadline: 金曜日までにご連絡ください (“Please contact me by Friday”). For counted days, 三日以内に sounds more natural than 三日までに.

Does 連絡 imply phone, email, or any contact?

連絡 is generic: phone, email, message, etc. If you want to be specific, say:

Phone: 電話してください / お電話ください
Email: メールしてください / メールを送ってください
Message: メッセージをください

I see 以内 (いない), which sounds like “is not there” (いない). Is that a problem?

They’re homophones but different words:

以内 (いない) = within, not exceeding.
いない (hiragana) = not existing/absent (negative of いる). Kanji and context prevent confusion in writing; context handles it in speech.