natsu wa boushi o kabutte soto o aruku you ni shite iru.

In summer I make sure to wear a hat when I walk outside.

Breakdown of natsu wa boushi o kabutte soto o aruku you ni shite iru.

• は: topic particle
• を: direct object particle
• 夏なつ: summer
• 歩あるく: to walk

Questions & Answers about natsu wa boushi o kabutte soto o aruku you ni shite iru.

Why does 夏 use the particle は? Why not 夏に or just 夏?

は here is the topic marker.

• 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?