iki wo tomemasita.

It is read いきをとめました.

• 息 = いき
• を = o (the particle を is pronounced お in modern Japanese)
• 止めました = とめました

息 means breath or breathing.

In this expression, 息を止める, it refers to stopping your breath for a moment, so in natural English it often becomes hold one’s breath.

を marks the direct object of the verb.

So in 息を止めました:

• 息 = the thing being affected
• を = object marker
• 止めました = stopped

Literally, it is something like stopped the breath, but in natural English this usually means held one’s breath.

The dictionary form is 止める.

This sentence uses the polite past form:

• 止める = to stop
• 止めました = stopped

So 止めました is the -masu past form of 止める.

This is a very common question because 止める and 止まる are related but different.

• 止める = to stop something (transitive)
• 止まる = to stop / to come to a stop (intransitive)

In 息を止めました, the speaker is treating breath as the thing being stopped, so Japanese uses the transitive verb 止める.

That is why it is:

• 息を止める = to hold one’s breath

Not:

• 息が止まる in this context

However, 息が止まる does exist, but it usually means something like one’s breath stops on its own, often from shock, surprise, or fear.

Usually it means held one’s breath.

That is the most natural interpretation in many everyday contexts.

• 息を止める = to hold one’s breath

If you translated it too literally as stopped breathing, it could sound stronger or more serious in English than the Japanese often intends.

So unless the context is medical or dramatic, held his/her/my breath is often the best English translation.

Because 止めました is past tense.

It shows that the action already happened:

• 息を止めます = will hold / hold one’s breath
• 息を止めました = held one’s breath

Depending on context, it could mean:

• I held my breath
• She held her breath
• He held his breath
• They held their breath

Japanese often leaves that unstated.

It is omitted.

Japanese very often leaves out the subject when it is clear from context. So 息を止めました by itself does not tell you who did it.

The subject has to be understood from the situation or from earlier sentences.

So this could mean:

• I held my breath
• She held her breath
• He held his breath
• We held our breath

Context decides.

Yes, very much so.

Even though each part has a straightforward meaning, 息を止める is a common expression that learners should remember as a chunk:

• 息を止める = to hold one’s breath

This is more natural than trying to understand it only word by word every time.

No. In normal Japanese writing, you would usually write it without spaces:

息を止めました。

Spaces are often added only in teaching materials to make the parts easier for learners to see.

Yes. The verb とめる can sometimes be written in different ways depending on meaning, such as 止める or 停める.

Here, 止める is the correct and natural choice because it means stop in the sense of stopping an action or movement.

So:

• 息を止める = correct

You would not normally use 停める here, because that form is more for things like parking or bringing a vehicle to a stop.