kyouto ni iku toki, eki de bentou wo kaimasu.

Because 行く often marks its destination with に.

• 京都に行く = go to Kyoto
• Here, に shows the place you are heading toward.

You can also often use へ with 行く:

• 京都に行く
• 京都へ行く

Both mean almost the same thing here. Very roughly:

• に focuses more directly on the destination
• へ can feel a little more like toward

In this sentence, に is completely natural and very common.

Yes, it could.

• 京都に行くとき
• 京都へ行くとき

Both are grammatical and natural. In everyday use, the difference is small.

A simple way to think about it:

• に = destination
• へ = direction toward that destination

With 行く, both are common, and learners should understand both.

とき means when or at the time of.

So:

• 京都に行くとき = when I go to Kyoto / when going to Kyoto

It connects the idea of going to Kyoto with the main action buying a bento at the station.

Structure:

• Verb + とき = when doing ...
• Noun + の + とき = when it is ...
  • for example, 学生のとき = when I was a student

This is a very common and important question.

In Japanese, the tense before とき helps show whether that action is completed yet at the time of the main action.

• 行くとき = when going / before or as one goes
• 行ったとき = when I went / after going / when I had gone

In your sentence:

• 京都に行くとき、駅で弁当を買います。

This suggests the bento is bought before or as part of going to Kyoto—probably at the station before boarding.

Compare:

• 京都に行くとき、駅で弁当を買います。
  = I buy a bento when I go to Kyoto.

• 京都に行ったとき、駅で弁当を買いました。
  = I bought a bento when I went to Kyoto.
  This sounds like talking about a past trip.

So 行くとき is used because the trip to Kyoto has not been completed yet at the moment of buying.

Because で marks the place where an action happens.

• 駅で買います = buy at the station

Here, the action is 買います (buy), and 駅で tells you where that action takes place.

Compare:

• 駅で買います = buy at the station
• 駅に行きます = go to the station

So:

• に often marks destination
• で often marks location of an action

Because を marks the direct object of the verb.

Here, the verb is 買います (buy), and the thing being bought is 弁当.

So:

• 弁当を買います = buy a bento

This is one of the most basic Japanese patterns:

• Noun + を + Verb

Examples:

• 本を読みます = read a book
• 水を飲みます = drink water
• 弁当を買います = buy a bento

弁当 means a boxed meal, often called bento in English too.

It usually refers to a meal packed in a container, often with rice and side dishes. In Japan, bentos are commonly sold:

• at stations
• at convenience stores
• at supermarkets
• in department stores

In this sentence, 駅で弁当を買います gives a very natural image: buying a boxed meal at the station before a trip.

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

So the sentence could mean:

• I buy a bento at the station when I go to Kyoto
• We buy a bento...
• He/She buys a bento...

Who it refers to depends on context.

In English, we usually need a subject. In Japanese, if the subject is already understood, it is often left out.

That is why many Japanese sentences sound shorter than their English equivalents.

It can be understood either way depending on context, but by itself it often sounds like a habitual or general statement.

• 京都に行くとき、駅で弁当を買います。 can mean
  When I go to Kyoto, I buy a bento at the station.

This sounds like something the speaker typically does.

But in the right context, it could also refer to a future plan:

• 明日京都に行くとき、駅で弁当を買います。 = Tomorrow, when I go to Kyoto, I’ll buy a bento at the station.

Japanese non-past forms like 行く and 買います can express both present/future and habitual meaning.

買います is the polite -ます form of 買う.

• casual: 買う
• polite: 買います

Japanese sentences are often shown in polite form in textbooks because it is useful and safe in many situations.

So this sentence in casual style would be:

• 京都に行くとき、駅で弁当を買う。

Notice something important:

• even though the main verb is polite (買います),
• the verb before とき stays in plain form: 行く

That is normal.

Because grammar words like とき usually attach to the plain form, not the -ます form.

So you say:

• 行くとき = when going
• 食べるとき = when eating
• 行ったとき = when went
• 食べたとき = when ate

Not:

• 行きますとき
• 食べますとき

Those sound unnatural in standard modern Japanese.

This is a pattern learners need to get used to: even in polite sentences, subordinate parts often use plain forms.

It is not always strictly necessary, but it is very natural and helpful.

• 京都に行くとき、駅で弁当を買います。

The comma separates the time phrase 京都に行くとき from the main statement 駅で弁当を買います.

Without the comma, the sentence is still understandable:

• 京都に行くとき駅で弁当を買います。

But the comma makes it easier to read.

Yes, to some extent. Japanese word order is more flexible than English, as long as the particles stay correct.

For example, these are possible:

• 京都に行くとき、駅で弁当を買います。
• 駅で、京都に行くとき弁当を買います。
• 弁当を、京都に行くとき駅で買います。

However, not all versions sound equally natural. The original sentence is the most straightforward and natural.

A useful rule:

• the verb usually comes at the end
• phrases before it can move around more than in English
• particles like に, で, and を show each word’s role

No. It means when I go to Kyoto, not when I am in Kyoto.

If you want to say when I am in Kyoto, you would use a different expression, such as:

• 京都にいるとき = when I am in Kyoto
• 京都に行ったとき can sometimes mean when I went to Kyoto, depending on context

So:

• 行く = go
• いる = be/stay

That difference matters a lot here, because this sentence describes something done on the way / before going, not while already staying in Kyoto.