Learn Hardcore Japanese: gaisyutusuru mae ni kusuri wo baggu ni irete oku. - Before going out, I put the medicine in my bag in advance.

The form 入れておく combines the ‑て form of 入れる (“to put in”) with the auxiliary おく, which means “to do something in advance (for future convenience).” So 薬をバッグに入れておく isn’t just “put the medicine in the bag,” it carries the nuance “put the medicine in the bag ahead of time” or “make sure it’s in the bag before you go out.” By contrast, 入れる alone just states the action without implying any preparatory motive.

Why is 前 followed by に (as in 前に)?

When you use 前 (“before”) to indicate a point in time, you attach the particle に. 前に marks “the time before X happens.” In this sentence, 外出する前に means “before going out.” If you left off に, it wouldn’t correctly mark that time relationship.

What does 外出する mean?

外出する is a verb meaning “to go out” or “to leave home for an outing.” It’s a compound of 外出 (outing, going out) plus する (“to do”), making it “to go out.”

Why is 薬 marked with を?

薬 (“medicine”) is the direct object of the transitive verb 入れる (“to put in”). The particle を marks what is being put into the bag.

Why is バッグ followed by に?

In this sentence, バッグ (“bag”) is the destination or target of the action “put.” The particle に marks the location into which something is placed: “into the bag.”

Why is the sentence in the plain (dictionary) form instead of the polite form?

Japanese plain form is commonly used in writing, diaries, notes, or casual conversation. This example shows the basic grammar pattern, so it uses plain form. If you wanted to make it polite, you’d say:
外出する前に薬をバッグに入れておきます。

Could you say バッグに薬を入れておく instead of 薬をバッグに入れておく? How flexible is the word order?

Yes. Japanese word order is flexible as long as particles clearly mark each element’s role.

薬をバッグに入れておく (object before location)
バッグに薬を入れておく (location before object)
Both are grammatical. Switching can shift emphasis slightly (e.g. focusing on the bag first), but the overall meaning remains the same.

Could you say 外出前に instead of 外出する前に? Is there any difference?

Yes, you can shorten 外出する前に to 外出前に by turning 外出 into a noun. Both mean “before going out.”

外出前に is a bit more compact (common in everyday speech).
外出する前に is slightly more explicit since it uses the verb form.

Why is there no subject in this sentence? Who is doing the action?

Japanese often omits the subject when it’s clear from context. Here, the implied subject is “I” (or “we”). In full it would be:
（私は）外出する前に薬をバッグに入れておく。
but in natural Japanese you drop 私は when it’s obvious.

Why is the verb 入れておく used here rather than 持っていく, which also means “to take/bring” something?

入れておく emphasizes the preparatory step—“put in the bag ahead of time so it’s ready.” 持っていく simply means “take with you” or “bring along.” If you said 薬をバッグに入れて持っていく, you’d be clear you’re taking it, but the nuance of “setting it up in advance” comes through most directly with 入れておく.