watasi ha kyouto he iku node, ouhuku no kippu wo kaimasita.

は marks 私 as the topic of the sentence. It tells you that the sentence is about me.

So 私は means something like:

• As for me, ...
• I ...

In natural English, we usually just translate it as I, but the nuance is slightly different. Japanese is setting up 私 as the topic, not just marking it as the grammatical subject.

It could definitely be omitted if the speaker is already understood from context.

Japanese often leaves out subjects and topics when they are obvious. So this could also be:

• 京都へ行くので、往復の切符を買いました。

That would still naturally mean I bought a round-trip ticket because I’m going to Kyoto, assuming the speaker is talking about themself.

Using 私 makes it clearer or slightly more explicit.

へ marks direction: to / toward Kyoto.

So:

• 京都へ行く = go to Kyoto

With verbs of movement like 行く, both へ and に are often possible:

• 京都へ行く
• 京都に行く

Both are natural and usually very similar in meaning. Very roughly:

• へ emphasizes direction
• に can emphasize destination

In many everyday sentences, the difference is small.

Because 行く describes the planned action of going to Kyoto, not something that has already happened.

Japanese non-past forms often cover both:

• present
• future

So 京都へ行く here means I am going to Kyoto or I will go to Kyoto.

The sentence is saying:

• I bought a round-trip ticket because I’m going to Kyoto

The ticket purchase happened in the past, so 買いました is past. The trip may still be in the future, so 行く stays non-past.

ので means because or since.

In this sentence:

• 京都へ行くので、往復の切符を買いました。
• Because I’m going to Kyoto, I bought a round-trip ticket.

A very important nuance: ので often sounds a bit softer, more explanatory, and a little less direct than から.

So ので often gives the feeling of:

• explaining a reason
• giving background
• sounding polite or matter-of-fact

Both can mean because, but they do not always feel the same.

ので

• softer
• more explanatory
• often sounds more polite or less blunt

から

• more direct
• more assertive
• often feels more like a straightforward reason

So:

• 京都へ行くので、往復の切符を買いました。
• 京都へ行くから、往復の切符を買いました。

Both are correct, but ので sounds slightly more like calm explanation.

This is normal Japanese grammar.

Before ので, the verb appears in its plain form:

• 行くので
• not 行きますので in this kind of basic sentence

The politeness of the whole sentence is usually shown at the end, in the final verb:

• 買いました

So it is very common to have:

• plain form inside the sentence
• polite form at the end

That is exactly what is happening here.

往復 means round trip or going and returning.
切符 means ticket.

So:

• 往復の切符 = a round-trip ticket

The particle の connects nouns. It often works like:

• of
• a noun modifier
• something like an adjectival link

Here, 往復 is modifying 切符:

• a ticket for a round trip

を marks the direct object of the verb.

The thing being bought is:

• 往復の切符

So:

• 往復の切符を買いました
• bought a round-trip ticket

In other words, を shows what was bought.

Japanese very often puts the reason before the main action.

So the structure is:

• 京都へ行くので = because I’m going to Kyoto
• 往復の切符を買いました = I bought a round-trip ticket

This order is extremely natural in Japanese. English can also do this:

• Because I’m going to Kyoto, I bought a round-trip ticket.

Japanese frequently builds up context first and gives the main action afterward.

The comma separates the reason part from the main statement:

• 京都へ行くので、
• 往復の切符を買いました。

It helps readability and makes the pause clearer.

In Japanese, commas are often more flexible than in English, so sometimes you will see them and sometimes not. But here the comma is very natural because the sentence has two clear parts.

Yes, that is a very natural interpretation.

• 買いました tells us the buying already happened.
• 行く suggests the trip to Kyoto is still upcoming, planned, or not yet completed.

So the likely situation is:

• The speaker is going to Kyoto.
• Because of that, they bought a round-trip ticket.
• The purchase is finished; the trip may still be in the future.

Both words can mean ticket, but 切符 is a very common native-style word for transportation tickets such as:

• train tickets
• bus tickets
• some travel tickets

チケット is also common in Japanese, but it is often used for things like:

• concert tickets
• movie tickets
• event tickets

For travel, 切符 sounds very natural here.