hosuukei wo katta koto wo kikkake ni, hosuu wo kirokusi hazimeta.

歩数計 means pedometer or step counter.

It breaks down like this:

• 歩数 = number of steps
• 計 = meter, counter, measuring device

So literally, it is something like step-count meter.

歩数 means step count or number of steps.

In this sentence, the person is not saying they started walking. They are saying they started recording how many steps they take.

～ことをきっかけに means something like:

• using ... as a trigger
• after ... prompted me to
• taking ... as an opportunity

So:

• 歩数計を買ったことをきっかけに = using the fact that I bought a pedometer as the trigger / after buying a pedometer, that led me to...

The nuance is important: きっかけ is not just a neutral cause. It is more like a starting point or turning point that led to a new action.

こと turns the whole clause before it into a noun-like phrase.

• 歩数計を買った = bought a pedometer
• 歩数計を買ったこと = the fact of having bought a pedometer / buying a pedometer

Japanese often uses こと to nominalize an action so it can function as part of a larger structure.

Here, the whole event buying a pedometer becomes the trigger.

Because the buying happened first.

The sequence is:

1. bought a pedometer
2. that became the trigger
3. started recording step counts

So 買ったこと refers to the completed event having bought it.

Using the past form before こと is very common when talking about something that already happened:

• 日本へ行ったこと = the fact that I went to Japan / the experience of going to Japan
• 本を読んだこと = the fact that I read the book

They belong to different parts of the sentence.

1. 歩数計を買った

   • 歩数計を marks 歩数計 as the direct object of 買った
   • bought a pedometer

2. 歩数計を買ったことをきっかけに

   • this を is part of the pattern Nをきっかけに
   • taking N as a trigger/opportunity

So the first を belongs to 買う, and the second を belongs to the grammar pattern with きっかけ.

This is the verb 記録する combined with 始める.

• 記録する = to record
• 記録し = the ます-stem of 記録する
• 始めた = started

So:

• 記録し始めた = started recording

This is a very common pattern:

• 読み始めた = started reading
• 食べ始めた = started eating
• 勉強し始めた = started studying

In your sentence, spaces are shown for learning purposes, but in normal writing it would usually be written as 記録し始めた.

Both are possible in some contexts, but they are slightly different.

• 記録し始めた = started recording
  • focuses on starting the action
• 記録を始めた = started the record/record-keeping
  • can sound a little more like began the practice/system of keeping records

In this sentence, 記録し始めた is very natural because it clearly describes the action itself.

The subject is omitted, which is very normal in Japanese.

Depending on context, it could mean:

• I started recording my step count
• he/she started recording their step count
• they started recording their step count

Japanese often leaves out the subject when it is already understood from context.

In ～をきっかけに, the に helps connect the trigger to the main action.

You can think of it as part of a set expression:

• Nをきっかけに、... = using N as a trigger, ...

So it is best learned as one chunk:

• をきっかけに

Not exactly.

Because usually just gives a reason or cause.

きっかけ gives the idea of:

• a trigger
• a prompt
• a turning point
• something that led to a new action or change

So this sentence is not just saying:

• Because I bought a pedometer, I recorded my steps

It more specifically suggests:

• Buying a pedometer was what got me to start recording my steps

That nuance is why きっかけ is used.

Yes, that is a very natural translation.

Just keep in mind that after is a smooth English rendering, but the Japanese has a slightly more specific nuance:

• not just after
• but prompted by that event / as a result of that becoming a trigger

So after buying a pedometer is fine in English, but buying a pedometer prompted me to start recording my step count is closer to the nuance.

Usually, yes.

始めた means started, so it naturally suggests the action began at that time and likely continued, at least for some period.

In this sentence, the idea is:

• the person began the habit of recording step counts

It does not explicitly say how long they continued, but it strongly suggests more than a one-time action.