kanposhita koto o kirokuhyou ni kaku.

I write on the record sheet that I completed the walk.

Breakdown of kanposhita koto o kirokuhyou ni kaku.

• を: direct object particle
• 書かく: to write
• に: target particle
• こと: thing

Questions & Answers about kanposhita koto o kirokuhyou ni kaku.

Why is there a こと after 完歩した?

Here こと turns the whole action 完歩した into a noun-like phrase.

So:

• 完歩した = completed the walk / finished the course on foot
• 完歩したこと = the fact of having completed the walk

That noun-like phrase can then be marked by を and become the thing that is written.

A natural way to understand the structure is:

• [完歩したこと] を [記録表に] 書く
• Write the fact that one completed the walk on the record sheet

This use of こと is very common when Japanese wants to treat an action or event as a thing.

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Why is it 完歩したことを and not just 完歩を?

Because the sentence is not talking about writing the word 完歩 itself. It is talking about writing that someone completed the walk.

Compare:

• 完歩を書く would sound like writing the word or label 完歩
• 完歩したことを書く means writing down the fact/event of having completed it

So こと is needed because what is being written is not just a vocabulary item, but a whole piece of information.

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Why is した in the past form, even though the sentence ends with 書く?