tosyokan ni yotta toki, uketuke no hito ni nihongo de hanasikakemasita.

Here, 図書館に寄った means stopped by the library.

With 寄る, the particle に marks the place you stop at. So:

• 図書館に寄る = stop by the library
• コンビニに寄る = stop by the convenience store

This is different from just saying 図書館に行く, which simply means go to the library.
寄る has the nuance of dropping in or stopping by on the way.

寄った is the past plain form of 寄る.

• dictionary form: 寄る
• past plain form: 寄った

In this sentence, 寄った modifies とき:

• 寄ったとき = when I stopped by

Japanese often uses a plain form before nouns, and とき acts like a noun meaning time / when.

The past form here does not necessarily mean the whole sentence is in the distant past. It shows the action relative to とき.

This is a very important grammar point.

• 寄るとき = when stopping by / when about to stop by
• 寄ったとき = when I stopped by / when I had stopped by

The tense before とき depends on whether the action has happened yet at that time.

So:

• 図書館に寄るとき would suggest when I go to stop by the library
• 図書館に寄ったとき means when I had stopped by the library / when I was at the point of having stopped by

In natural English both may sometimes become just when I stopped by the library, but Japanese makes this timing distinction more clearly.

とき means time or when.

It turns the preceding clause into a time expression:

• 図書館に寄ったとき = when I stopped by the library

So the structure is:

• [clause] + とき = when [clause]

Examples:

• 日本に行ったとき = when I went to Japan
• 暇なとき = when I’m free
• 子どものとき = when I was a child

Yes, ときに is possible.

• 図書館に寄ったとき、...
• 図書館に寄ったときに、...

Both are natural. The version without に is very common and slightly more straightforward here.

Adding に can sometimes make the time expression feel a little more explicit, but in many sentences the difference is small.

Literally, 受付の人 is the reception person.

More naturally in English, it means something like:

• the person at the reception desk
• the receptionist
• the front desk staff member

The structure is:

• 受付 = reception, front desk
• の = linking particle, like of or at
• 人 = person

So 受付の人 means the person connected with the reception/front desk.

This に marks the person that the action is directed toward.

• 受付の人に話しかけました = I spoke to / addressed the person at reception

With 話しかける, the person you speak to is marked by に.

So the sentence has two different に particles doing two different jobs:

• 図書館に = location you stopped by
• 受付の人に = person you spoke to

This is very normal in Japanese.

話しかける means to speak to someone, especially with the nuance of starting to talk to them or addressing them.

• 話す = speak, talk
• 話しかける = speak to someone / start talking to someone

So:

• 受付の人に話しました can sound more like I talked to the receptionist
• 受付の人に話しかけました emphasizes that you approached them and said something to them

In this sentence, 話しかけました is a very natural choice because the speaker is initiating speech with the receptionist.

The particle で is used here to show the language used.

• 日本語で話しかけました = spoke to them in Japanese

So で means something like using or by means of.

Examples:

• 英語で話す = speak in English
• 日本語で書く = write in Japanese
• ペンで書く = write with a pen

If you used 日本語を, that would usually make Japanese the direct object, which does not fit this sentence.

Yes, the subject is omitted.

Japanese often leaves out the subject when it is clear from context. In a sentence like this, the omitted subject is most likely I:

• (私は) 図書館に寄ったとき、受付の人に日本語で話しかけました。

Japanese does this constantly. If the subject is obvious, it is usually more natural to omit it.

Japanese is basically a subject-object-verb language, so the main verb usually comes at the end.

This sentence is structured like:

• 図書館に寄ったとき、 = when I stopped by the library,
• 受付の人に = to the person at reception
• 日本語で = in Japanese
• 話しかけました = spoke / addressed them

All the details come before the final verb. This is one of the biggest differences from English word order.

の links two nouns.

In 受付の人, it connects:

• 受付 = reception/front desk
• 人 = person

So the whole phrase means the person of the reception, or more naturally, the person at reception.

This noun + の + noun pattern is extremely common:

• 日本の文化 = Japanese culture / the culture of Japan
• 学校の先生 = school teacher / teacher at a school
• 友だちの車 = my friend’s car

In this sentence, it is physical: stopping by the library.

But 寄る is broader than just go to. It often has the feeling of:

• dropping in
• stopping by briefly
• coming by on the way somewhere

So it often suggests a casual or secondary visit, not necessarily the main destination.

For example:

• 帰りにスーパーに寄った = I stopped by the supermarket on the way home
• ちょっと店に寄る = I’ll just stop by the shop

So compared with 行く, 寄る gives a more specific nuance.

It is polite overall because the main verb is in ます form:

• 話しかけました

At the same time, the clause before とき uses the plain form:

• 寄った

This combination is completely normal. In Japanese, subordinate clauses often use plain forms even in polite sentences.

So this is a natural polite sentence, not a mismatch.

A casual version would usually change the final verb to plain past:

• 図書館に寄ったとき、受付の人に日本語で話しかけた。

Everything else can stay the same.

So the difference is mainly:

• polite: 話しかけました
• casual: 話しかけた

Both mean the same thing; the difference is tone and level of formality.