sukkari kuraku natte ita node, kaeri ha basu ni notta.

Because 暗い is an -i adjective, and when an -i adjective is used before なる to mean become ..., it changes to its -ku form.

• 暗い = dark
• 暗くなる = to become dark

So:

• 暗い by itself describes something: It is dark
• 暗くなる means a change: It becomes dark

This is a very common pattern:

• 寒い → 寒くなる = become cold
• 早い → 早くなる = become early / become fast
• 明るい → 明るくなる = become bright

すっかり means something like completely, entirely, or thoroughly.

In this sentence, it emphasizes that it was not just a little dark—it had become fully dark.

So:

• 暗くなった = it got dark
• すっかり暗くなった = it got completely dark / it was totally dark by then

It often adds a feeling that a change has been completed.

Other examples:

• すっかり忘れた = I completely forgot
• すっかり元気になった = recovered completely / became totally well

なっていた is the past form of 〜ている, and here it shows the state resulting from a change.

• 暗くなった = it became dark
• 暗くなっていた = it had become dark / it was already dark

The difference is subtle but important:

• なった focuses more on the moment of change
• なっていた focuses on the condition that was already true at that time

In this sentence, the speaker is explaining why they took the bus on the way back. By that point, it was already dark, so なっていた fits naturally.

No, 〜ている does not always mean an ongoing action.

It has several common uses, including:

1. An action in progress

   • 食べている = is eating

2. A resulting state

   • 結婚している = is married
   • 知っている = knows
   • 暗くなっている = has become dark / is dark

With change verbs like なる, 〜ている often describes the state after the change.

So here, 暗くなっていた means the surroundings had already become dark, not that darkness was still gradually happening.

Both ので and から can mean because, but they have different tones.

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

So:

• 暗くなっていたので、帰りはバスに乗った。
  = Since it had gotten dark, I took the bus on the way back.

This sounds like a calm explanation of the reason.

If you used から instead, it would still be grammatically correct, but the nuance would be slightly more direct:

• 暗くなっていたから、帰りはバスに乗った。

Both are natural. ので just fits well when giving a reason in a gentle, matter-of-fact way.

帰り is a noun meaning the way back, the return trip, or returning home depending on context.

In this sentence:

• 帰りは = as for the way back / on the way back

It does not mean the act of going home in general as a verb. It is a noun referring to the return part of the trip.

Common related expressions:

• 行き = going / the way there
• 帰り = returning / the way back

For example:

• 行きは電車で、帰りはバスで行った。
  = I went by train, and on the way back I went by bus.

The は marks 帰り as the topic.

So 帰りは means something like:

• as for the way back
• on the return trip
• when it came to going back

It can also create a mild sense of contrast, even if the other side is not said explicitly. The sentence suggests something like:

• Maybe the outward trip was different
• But as for the return, the speaker took the bus

So 帰りはバスに乗った has a nuance like I took the bus on the way back (at least on the way back).

The verb 乗る uses に for the vehicle or thing you board/ride.

So:

• バスに乗る = ride/get on a bus
• 電車に乗る = ride/get on a train
• 自転車に乗る = ride a bicycle

This is just the normal particle pattern for 乗る.

English speakers often expect を, but with 乗る, Japanese uses に because the vehicle is treated more like the destination or target of boarding.

Because 乗る normally takes に, not で, for the vehicle.

• バスに乗る = to get on / ride a bus

Using で with 乗る would sound unnatural.

However, で is used with other verbs to show the means of transportation:

• バスで帰った = I went back by bus
• 電車で行った = I went by train

So both of these are natural, but they use different verbs:

• バスに乗った = got on / rode the bus
• バスで帰った = returned by bus

The sentence chose 乗った, which focuses specifically on taking the bus.

Yes, Japanese often omits subjects when they are clear from context.

In this sentence:

• The thing that became dark is the surroundings / the sky / outside
• The person who rode the bus is the speaker, or the speaker’s group, depending on context

Japanese usually does not need to say it or I if the meaning is obvious.

So English might need:

• It had gotten completely dark, so I took the bus on the way back.

But Japanese naturally leaves those unstated.

Japanese very often puts the reason first and the main result after it.

So the structure is:

• すっかり暗くなっていたので = since it had gotten completely dark
• 帰りはバスに乗った = I took the bus on the way back

This order is very natural in Japanese. It prepares the listener for the conclusion.

Japanese frequently uses this pattern:

• 雨だったので、行かなかった。
  = Because it was raining, I didn’t go.

• 忙しかったので、宿題ができなかった。
  = Because I was busy, I couldn’t do my homework.

Yes. In natural English, both can work depending on how closely you want to reflect the Japanese.

Because the Japanese uses 暗くなっていた, the literal idea is:

• it had become dark
• it had already gotten dark

But in smoother English, people often say:

• It was already completely dark, so I took the bus on the way back.

That translation captures the meaning well, even if it is a little less literal.

It is not a fixed phrase, but it uses a very common pattern:

adverb + adjective/adjective-change + なっていた + ので

More generally:

• [something] + くなる = become ...
• [something] + くなっている = has become ... / is now ...
• [something] + くなっていたので = because it had become ...

Examples:

• 寒くなっていたので、コートを着た。
  = Since it had gotten cold, I put on a coat.

• 遅くなっていたので、急いだ。
  = Since it had gotten late, I hurried.

So this sentence is a good example of a very useful grammar pattern.