henna oto ga sita node, sugu kikai wo tomemasita.

変 is a na-adjective, not an i-adjective. That means it needs な before a noun.

• 変な音 = a strange sound
• not 変い音

This is similar to:

• 静かな部屋 = a quiet room
• 便利な道具 = a convenient tool

Even though 変 ends in a sound that may look short and simple, it does not behave like an i-adjective.

Literally, 音がした means a sound did / occurred, but in natural English it usually means I heard a sound or there was a sound.

In Japanese, する is often used for things that are sensed:

• 音がする = a sound is heard / there is a sound
• いい匂いがする = there is a nice smell / it smells good
• 味がする = there is a taste / it tastes like ...

So 変な音がした means a strange sound was heard or there was a strange sound.

が marks 音 as the thing that occurred or was perceived.

• 音がした = a sound happened / was heard

This pattern is very common with sensory expressions like 音がする, 匂いがする, and 気がする.

If you are thinking in English, it may feel like I heard a sound, so you might expect a direct object. But Japanese often expresses this as a sound occurred rather than I heard a sound.

した is the past tense of する.

• 音がする = a sound happens / is heard
• 音がした = a sound happened / was heard

Since the sentence describes a completed event in the past, した is used. The strange sound happened first, and then the speaker stopped the machine.

ので means because, since, or so. It connects the reason to the result.

• 変な音がしたので、すぐ機械を止めました。
• Because there was a strange sound, I immediately stopped the machine.

It often sounds a little more neutral, explanatory, or gentle than から.

Both can mean because, but they have a slightly different feel.

• ので often sounds more explanatory, objective, or softer
• から can sound more direct, personal, or assertive

Compare:

• 変な音がしたので、止めました。

  • Because there was a strange sound, I stopped it.
  • Slightly calmer/more explanatory

• 変な音がしたから、止めました。

  • Because there was a strange sound, I stopped it.
  • Slightly more direct

In many everyday cases, both are possible.

すぐ means immediately / right away. It modifies the action 止めました.

So the structure is:

• 変な音がしたので = because there was a strange sound
• すぐ = immediately
• 機械を止めました = stopped the machine

Japanese adverbs like すぐ often come before the verb or before the phrase containing the verb.

These two mean different things.

• 機械を止めました = I stopped the machine
• 機械が止まりました = the machine stopped

The sentence uses 止めました, the transitive verb, because someone actively stopped the machine.

This is an important pair:

• 止める = to stop something
• 止まる = to stop, to come to a stop

So を is used because 機械 is the direct object of 止める.

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

So 機械を止めました does not explicitly say I stopped the machine, but that is the natural interpretation in many situations.

Depending on context, it could also mean:

• we stopped the machine
• someone stopped the machine

But if this is a normal statement about what the speaker did, English usually translates it as I stopped the machine.

Because the whole sentence is talking about a past event.

Sequence:

1. 変な音がした = a strange sound happened / was heard
2. 機械を止めました = the machine was stopped

Both actions are completed, so both verbs are in the past tense.

Yes. The final verb 止めました is in polite -ます form.

Notice, though, that the verb before ので is plain past:

• したので
• not しましたので in this sentence

This is normal. In Japanese, the verb before a connector like ので is often in plain form even when the sentence ends politely.

So this mixture is natural:

• 変な音がしたので、すぐ機械を止めました。

Yes. 変な can mean strange, odd, funny, weird, or abnormal, depending on context.

So 変な音 could mean:

• a weird sound
• an unusual sound
• a suspicious sound
• an abnormal machine noise

In a machine-related sentence, it often suggests something may be wrong.

The comma helps separate the reason part from the main action part:

• 変な音がしたので、 = because there was a strange sound,
• すぐ機械を止めました。 = I immediately stopped the machine.

Japanese commas are often used more flexibly than English commas. This comma is natural and helps readability, but punctuation in Japanese is not always as rigid as in English.