kanposita koto wo kirokuhyou ni kaku.

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.

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.

Because the completion happened before the writing.

• 完歩した = completed
• 書く = write

So the sentence describes this sequence:

1. First, the person completes the walk.
2. Then, they write that fact on the record sheet.

The past form inside a larger sentence does not mean the whole sentence is in the past. It just shows that the action 完歩する is already completed relative to 書く.

Plain-form 書く is often used in instructions, rules, labels, manuals, worksheets, and notices.

So even though 書く is not an imperative like 書け and not polite like 書きます, it can still function as an instruction:

• 記録表に書く。 = Write it on the record sheet.

This is very common in written directions. It sounds concise and neutral.

Not exactly.

完歩 means finishing a set walking course or walking the full distance. It has a sense of completing the walk, not just walking.

So:

• 歩いた = walked
• 完歩した = completed the walk/course on foot

This word is especially common in events like hikes, charity walks, school walking events, stamp rallies, or long-distance walking courses.

It is read かんぽ.

So the sentence begins:

• 完歩した = かんぽした

This is a good word to pay attention to, because かんぽ can refer to other words with different kanji in other contexts, but here 完歩 specifically means completing a walking course.

を marks the thing being written.

In this sentence, the direct object is not a physical object, but the content:

• 完歩したことを = the fact of having completed the walk

So:

• 何を書く？ = What do you write?
• 完歩したことを書く = Write that one completed the walk

This is a common pattern:

• 見たことを話す = talk about what you saw
• 思ったことを書く = write what you thought

With 書く, the particle に often marks the place where something is written.

So:

• 記録表に書く = write it on the record sheet

This is similar to:

• ノートに書く = write it in a notebook
• 紙に書く = write it on paper
• 欄に書く = write it in the box/space

Here に shows the destination or surface/location for the writing.

記録表 means something like:

• record sheet
• log sheet
• chart for recording information

The exact translation depends on the context. In this sentence, it is the form or sheet where the completion is recorded.

So 記録表に書く suggests filling in a designated record form, not casually writing on any piece of paper.

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

Depending on the situation, this could mean:

• You should write it on the record sheet.
• Participants write it on the record sheet.
• One writes it on the record sheet.

In English, we usually need to supply a subject, but Japanese often leaves it unstated if the context already makes it obvious.

This is especially common in instructions and notices.

Sometimes こと and の can both nominalize a verb phrase, but こと is the more natural choice here.

• 完歩したことを書く sounds natural and neutral.
• 完歩したのを書く is possible in some contexts, but it feels less standard here and more dependent on context.

In formal or informational writing, こと is often preferred for facts, events, or abstract content.

So for a sentence like this, こと is the safer and more natural form to learn.

Not necessarily.

Japanese often describes the content abstractly:

• 完歩したことを書く = write down the fact that the walk was completed

In practice, what gets written on the sheet might be:

• a check mark
• a date
• a participant name
• a note indicating completion
• a stamp or short entry

So the grammar refers to the information being recorded, not necessarily to writing a full sentence like in English.

It helps to divide it into chunks:

• 完歩したこと = the fact of having completed the walk
• を = marks that as the object
• 記録表に = on the record sheet
• 書く = write

So the whole structure is:

• [Thing to be written] を [place written to] に 書く

In this sentence:

• [完歩したこと] を [記録表] に 書く

That is a very useful pattern you can reuse with many nouns and verb phrases.