Learn Hardcore Japanese: depaato no naka ha hiroi node, mayowanai you ni iriguti no syasin wo totte oita. - The inside of the department store is spacious, so I took a photo of the entrance so I wouldn’t get lost.

デパート中 is not natural here; without の, it would more suggest “throughout all the department stores” or “all over department stores,” which is a different pattern. For “inside X,” you almost always say X の中.

What is the role of は in デパートの中は広いので? Why not が?

は marks the topic of the sentence: what we’re talking about.

デパートの中は広い – “As for the inside of the department store, (it) is spacious.”

If you used が:

デパートの中が広い – grammatically fine, but it emphasizes “it is the inside of the department store that is spacious,” focusing on デパートの中 as new or contrastive information.

With は, the speaker is more calmly presenting it as background information and then giving a result: because of that, they took a photo. It’s the natural choice for a clause giving a general reason like this.

What is the nuance of ので in 広いので compared to から? Could you say 広いから instead?

Both ので and から can mean “because / since,” and you could say:

デパートの中は広いから、迷わないように…

However, there’s a nuance:

ので sounds a bit softer, more formal, more explanatory. It’s often used in written language or polite speech when giving a reason in a relatively objective or gentle way.
から is more casual and direct. It can sound more like “because!” depending on tone and context.

In this sentence, 広いので feels like a polite, neutral explanation:
“Since the inside is spacious, (I did X).”

What does 迷わないように mean exactly, and why is ように used with the negative 迷わない?

迷わないように literally breaks down as:

迷わない – “not get lost” (negative plain form of 迷う)
ように – “so that,” “in such a way that,” “in order that”

Together:
迷わないように = “so that I don’t get lost” / “in order not to get lost.”

Using ように with the negative form is a common pattern to express a purpose of avoiding something:

遅れないように – so that (I) won’t be late
風邪をひかないように – so that (I) don’t catch a cold

So 迷わないように describes the goal or intention: avoiding getting lost.

What’s the difference between 迷わないように and 迷わないために here?

Both can translate as “so that I don’t get lost” / “in order not to get lost,” but:

ために is more purposeful, deliberate, like “for the sake of / for the purpose of.”
ように is often more soft / about the result, and is very common with things that are somewhat beyond full control (hopes, prevention, abilities, etc.).

In this sentence:

迷わないように sounds natural and gentle: “so that I (hopefully) won’t get lost.”
迷わないために is also correct, just a bit more “explicit-purpose” sounding: “for the purpose of not getting lost.”

Both are acceptable; ように is slightly more common in this kind of everyday context.

What does 入口の写真を撮っておいた mean exactly, and what is the function of ておいた here?

Breakdown:

入口 – entrance
の – of
写真 – photo
を – object marker
撮っておいた – “(I) took (and left it that way / in advance)”

撮っておいた is 撮る (“to take [a photo]”) in て-form plus おく in past tense (おいた).

～ておく has the nuance of:

Doing something in advance / beforehand,
Doing something and leaving it in a useful state for later.

So 撮っておいた here means:

“I took a picture (in advance, so that it would be useful later).”

Putting it all together:
入口の写真を撮っておいた = “I took a photo of the entrance (ahead of time).”

Why is the subject (“I”) not explicitly stated? How do we know it means “I took a photo…”?

In Japanese, the subject is often omitted when it’s clear from context.

In this sentence, the person who:

is worried about getting lost, and
takes a photo to prevent that

is naturally the speaker. So even without 私は, it’s understood as:

“Because the inside of the department store is spacious, I took a photo of the entrance so that I wouldn’t get lost.”

Adding 私は would be grammatically fine but usually unnecessary:
デパートの中は広いので、迷わないように入口の写真を撮っておいた。 ← perfectly natural as-is.

Why is it 入口の写真 (“photo of the entrance”) and not something like 入口を写真に撮る?

Both patterns exist, but X の写真 is the most straightforward and common way to say “a photo of X”:

入口の写真 – “a photo of the entrance”
富士山の写真 – a photo of Mt. Fuji
家族の写真 – a photo of (my) family

You can say things like:

入口を写真に撮る – literally “take the entrance into a photo,”
but this is more complex and less common in everyday speech.

So 入口の写真を撮る is the natural, simple pattern:

[X の写真] を撮る = take a photo of X.

Why is the verb 撮る (to take a photo) written with 撮 and not 取, and are both ever used?

Japanese has different kanji for different kinds of “taking”:

撮る – to take a photo / video / film
取る – to take, pick up, get, obtain (a wide range of meanings: take a book, take notes, take a day off, etc.)

For photos, you should use 撮る.
写真を取る is technically understandable but incorrect / non-standard; it looks like you’re using the wrong kanji. The correct standard form is:

写真を撮る – to take a photo.

Is the comma 、 after ので necessary, and does it change the meaning?

The comma in:

デパートの中は広いので、迷わないように入口の写真を撮っておいた。

is just a pause marker for readability, much like a comma in English. It does not change the grammar or meaning.

You could write it without the comma:

デパートの中は広いので迷わないように入口の写真を撮っておいた。

and it would still be correct. The comma simply makes it easier to see the boundary between the reason clause (～ので) and the main action that follows.