tenin ni nedan wo kiku mae ni, kousikisaito de sirabemasita.

It is read:

てんいん に ねだん を きく まえ に、こうしきサイト で しらべました。

Word readings:

• 店員 = てんいん
• 値段 = ねだん
• 聞く = きく
• 前 = まえ
• 公式サイト = こうしきサイト
• 調べました = しらべました

聞く can mean several related things: hear, listen, or ask. The particles and context tell you which meaning it has.

In this sentence:

• 店員に値段を聞く = ask the clerk the price

A useful pattern is:

• 人に + こと/ものを + 聞く = ask a person something
• 音楽を聞く = listen to music
• 話を聞く = hear/listen to a story

So here, because there is a person marked by に and a thing marked by を, 聞く clearly means ask.

Here, に marks the person you ask.

So:

• 店員に聞く = ask the clerk
• 先生に聞く = ask the teacher
• 友達に聞く = ask a friend

This is very common with 聞く when it means ask.

A good way to remember it is:

• 人に = the person information is directed to
• 物/内容を = the thing being asked

を marks the direct object, the thing being asked about.

So in:

• 店員に 値段を 聞く

the roles are:

• 店員に = to the clerk
• 値段を = the price

This is similar to English ask someone something:

• ask the clerk the price

In Japanese, the thing asked is usually marked with を.

Because the grammar pattern is:

• dictionary form + 前に = before doing
• noun + の + 前に = before a noun

So:

• 聞く前に = before asking
• 食べる前に = before eating
• 寝る前に = before sleeping

You do not use the past form before 前に in this pattern.

So:

• 聞く前に = correct
• 聞いた前に = incorrect

前に means before.

More literally:

• 前 = front / before
• に = marks the time point

So X前に、Y means:

• Y happens before X

In this sentence:

• 店員に値段を聞く前に、公式サイトで調べました。
• The checking happened before the asking.

It is the same particle in form, but it has a different job.

• 店員に: に marks the person being asked
• 前に: に marks a time expression, meaning at/before that point

This is very common in Japanese: the same particle can have different functions depending on the pattern.

So the two に's are not doing the same thing here.

Because で marks the place, medium, or means used for an action.

Here, 公式サイトで調べました means the speaker used the official website as the place/tool/platform for checking.

Compare:

• 図書館で調べる = look it up at the library
• インターネットで調べる = look it up on the internet
• 公式サイトで調べる = look it up on the official website

Using に here would sound unnatural with 調べる in this meaning.

Yes, but the meaning changes a bit.

• 公式サイトで調べました = I looked it up on the official website
• 公式サイトを調べました = I investigated/searched the official website itself

So で focuses on the website as the place or tool used to find information, while を makes the website the direct object of 調べる.

In this sentence, で is the more natural choice.

The object is omitted because it is understood from context.

Earlier in the sentence, we already have 値段. So after 公式サイトで調べました, Japanese naturally leaves out the repeated object.

The full version could be:

• 店員に値段を聞く前に、公式サイトで値段を調べました。

That is grammatical, but Japanese often omits repeated information when it is obvious.

Japanese often omits the subject when it is clear from context.

In this sentence, the understood subject is probably I, so English needs a subject, but Japanese does not.

You could say:

• 私は店員に値段を聞く前に、公式サイトで調べました。

But in many situations, 私は would be unnecessary and sound a little more explicit than needed.

By itself, 店員 does not clearly show singular or plural. Japanese nouns usually do not mark that distinction the way English does.

So 店員 could mean:

• a clerk
• the clerk
• sometimes even store staff, depending on context

In this sentence, most people will understand it as a clerk or the clerk.

値段 is very natural in everyday speech for price.

• 値段 = everyday, conversational
• 価格 = more formal, technical, business-like

So with a store clerk, 値段 sounds very natural:

• 店員に値段を聞く

You could use 価格, but it would sound a bit more formal:

• 店員に価格を聞く

For normal conversation in a shopping context, 値段 is usually the better choice.