Learn Hardcore Japanese: natu ha bousi wo kabutte soto wo aruku you ni site iru. - In summer I make sure to wear a hat when I walk outside.

夏は … = As for summer, / In (the) summer, (talking about a general habit in that season)
It sets 夏 (summer) as the context in which the rest of the sentence is true.

If you said:

夏に帽子をかぶって外を歩く。
This sounds more like “(I) walk outside wearing a hat in the summer” as a plain statement of when it happens, without strongly highlighting “summer” as the topic of discussion.

Using 夏は makes it feel like:

“In summer, I (as a rule) …”
It suits a habitual / general statement very well.

So 夏は = “As for summer / In summer (speaking generally).”

What exactly does 帽子をかぶって mean, and how does the て-form work here?

帽子をかぶって is:

帽子 – hat
を – object marker
かぶる – to put on / wear on the head
かぶって – て-form of かぶる

The て-form here links this action (wearing a hat) to the next verb (歩く):

帽子をかぶって外を歩く
≈ “walk outside wearing a hat”

In this kind of sentence, verb-て can mean:

Manner / accompanying state:
- Doing A while doing B → “walk outside with a hat on”
Or sometimes sequence:
- Do A, then do B → “put on a hat and then walk outside”

In context, this is clearly the first:
“I walk outside with a hat on / wearing a hat.”

Why is the verb かぶる used for a hat and not きる or something else?

Japanese has different verbs for “to wear/put on” depending on where you wear the item:

かぶる – for things worn on the head
- 帽子をかぶる – wear/put on a hat
かける – for things you hang on something, especially on the face
- めがねをかける – wear glasses
する – for accessories, small attached items
- ネクタイをする – wear a tie
- イヤリングをする – wear earrings
はめる / つける – for rings, gloves, etc.
- 指輪をはめる – put on a ring
はく – for things worn on the lower body / feet
- ズボンをはく – wear pants
- 靴をはく – wear shoes
きる – for things worn on the upper body / torso
- シャツをきる – wear/put on a shirt
- コートをきる – wear a coat

Since a hat goes on your head, the natural verb is かぶる:

帽子をかぶって = “wearing a hat.”

Why is 外 marked with を (外を歩く) instead of で (外で歩く)? What’s the difference?

With 歩く, both 外を歩く and 外で歩く are possible, but they have slightly different feels:

外を歩く
- を marks the path / area you move through.
- Emphasis: moving through or along the outside.
- Similar to: 公園を歩く (walk through the park), 道を歩く (walk along the road).
外で歩く
- で marks the location where the action takes place.
- Emphasis: the place where you are walking is “outside,” as opposed to, say, inside.

In practice:

外を歩く feels more natural when talking about walking around outside as an activity, especially as a route/habit.
外で歩く is grammatically fine, but you’ll more often see 外を歩く for this standard expression.

So here, 外を歩く ≈ “walk outside / walk around outside (as an activity).”

What does ～ようにしている mean in this sentence?

～ようにしている after a verb in dictionary form expresses the idea of:

“make sure to …”
“try to … (as a habit)”
“make it a rule to …”

Structure here:

歩く – to walk
歩くようにする – to try to walk / to make sure to walk
歩くようにしている – I am in the state of always trying to walk (i.e., I’ve made it a habit).

So:

夏は帽子をかぶって外を歩くようにしている。
≈ “In summer, I make sure to walk outside wearing a hat.”
≈ “In summer, I try to always walk outside with a hat on.”

Important nuance:
It doesn’t just say what you do, but that you consciously try / make an effort to do it as a habit.

How is 歩くようにしている different from just 歩いている or 歩く?

They express different nuances:

歩く (plain dictionary form)
- Just states the action generally.
- 夏は帽子をかぶって外を歩く。
  → “In summer, I walk outside wearing a hat.”
  (Simple statement of a fact, no emphasis on “trying.”)
歩いている (ている form)
- Can mean “am walking” (right now) or “habitually walk.”
- 夏は帽子をかぶって外を歩いている。
  → More like “These days, in summer I’ve been walking outside wearing a hat.”
  → Describes a current ongoing pattern, but not particularly “on purpose.”
歩くようにしている
- Adds intention/effort: “I make sure to / I try to walk.”
- Suggests a self-imposed habit or rule.

So ようにしている = habit + conscious effort, which is not expressed by just 歩く or 歩いている.

Who is the subject of this sentence? There is no “I” written anywhere.

Japanese often omits the subject when it is clear from context.

In this sentence:

There is no explicit 私 or 僕, but the meaning naturally reads as “I”:
- 夏は帽子をかぶって外を歩くようにしている。

Because:

It talks about a personal habit (“I make sure to…”).
Usually, when someone describes their own lifestyle/effort in conversation, the implied subject is the speaker.

If context were different, it could refer to someone else (e.g., “My grandmother, in summer, makes sure to walk outside wearing a hat”), but then that person would usually be introduced first:

祖母は夏は帽子をかぶって外を歩くようにしている。
“My grandmother, in summer, makes sure to walk outside wearing a hat.”

What kind of “tense” is している here? Does it mean “I am trying right now” or “I usually try”?

～ている in Japanese covers ongoing states and repeated habits, not just “right now” progressive.

Here, with 歩くようにしている:

It describes a habitual, ongoing situation:
- “I (generally) make sure to walk …”
- It’s about what you regularly do during summer, not just at this very moment.

So in natural English, we translate it as simple present:

“In summer, I make sure to walk outside wearing a hat.”
Not “I am making sure to walk…” in the sense of only right now.

Can the word order be changed, like 夏は外を帽子をかぶって歩くようにしている? Is that okay?

Japanese word order is more flexible than English, but there are natural preferences.

Original:

夏は　帽子をかぶって　外を歩く　ようにしている。

Here, 帽子をかぶって is closely connected to 歩く (“walk wearing a hat”).
Breaking up that relationship too much can sound unnatural or harder to parse.

夏は外を帽子をかぶって歩くようにしている。
This is grammatically not wrong, but it is awkward and unnatural.
Listeners might have to pause to understand the grouping.

More natural patterns keep related pieces together, e.g.:

夏は、外を、帽子をかぶって歩くようにしている。 (with commas/pauses)
夏は帽子をかぶって外を歩くようにしている。 (original)

So:
Yes, word order is somewhat flexible, but for clarity and naturalness, it’s best to leave it as in the original sentence.