wakaranaikoto ga atta node, sono ba de kakariin ni kikimasita.

It literally means a thing that is not understood.

Breakdown:

• 分からない = do not understand / is unclear
• こと = thing / matter

So 分からないこと means:

• something I didn’t understand
• something unclear
• a question / an issue I wasn’t sure about

In English we usually translate it more naturally than literally.

This is a very common question.

In Japanese, a plain non-past form like 分からない can describe a state that existed at that time. Here it means something like:

• there was something I didn’t understand
• there was something unclear to me

Using 分からなかったこと is not impossible in some contexts, but here 分からないことがあった is the more natural way to say there was something I didn’t understand.

So:

• 分からないことがあった = there was something unclear
• 分からなかったことがあった = there was something that I ended up not understanding / failed to understand

The first is the normal choice here.

こと turns the idea 分からない into a noun-like expression.

Without こと, 分からない is just not understand / unclear.
With こと, it becomes the thing that is unclear or something I don’t understand.

This is very common in Japanese:

• 知らないこと = something I don’t know
• 心配なこと = something I’m worried about
• 必要なこと = something necessary

So こと is helping package the whole idea into a matter / a thing.

Because 分からないこと is the thing that existed.

The verb ある means to exist / to be for non-living things.
Its past form is あった.

So:

• 分からないことがあった = there was something I didn’t understand

The thing that exists is usually marked by が.

You could think of it as:

• X が あった = there was X

It is made from:

• あった = there was
• ので = because / since

So:

• 分からないことがあったので = because there was something I didn’t understand

This whole first part gives the reason for what happens next.

Both can mean because, but the nuance is slightly different.

• ので sounds a bit softer, calmer, and more explanatory
• から sounds a bit more direct and assertive

So here:

• 分からないことがあったので、… sounds natural and polite
• 分からないことがあったから、… is also correct, but slightly more conversational/direct

In polite explanations, ので is very common.

その場 literally means that place or the spot.

In many cases, その場で means:

• right there
• on the spot
• then and there

So it does not just mean physical location. It often carries the idea of dealing with it immediately at that location.

Examples of the nuance:

• その場で決める = decide on the spot
• その場で払う = pay right there
• その場で聞きました = asked right there / on the spot

Because で marks the place where an action happens.

Here, the action is 聞きました (asked), and it happened at that spot.

Compare:

• その場に = to that place / at that place as a point of existence or destination
• その場で = at that place, where the action is performed

Since asking is an action, で is the natural choice.

係員 means something like:

• staff member
• attendant
• clerk
• person in charge

It is a fairly general word for an employee or official helper in a public or organized setting, such as:

• a station
• an event venue
• a counter
• a museum
• an office

It is a bit more formal or impersonal than some other words:

• 店員 = store clerk
• スタッフ = staff
• 社員 = employee
• 係員 = attendant/person in charge/staff member in a facility or service context

Because 聞く can mark the person you ask with に.

So:

• 係員に聞きました = I asked the staff member

The person receiving the question is marked by に.

If you include the thing you ask about, that thing may take を:

• 係員に道を聞きました = I asked the staff member for directions
• 係員にそのことを聞きました = I asked the staff member about that matter

So in this sentence:

• 係員に = to the staff member
• the content of the question is omitted because it is already understood from context

Yes. The thing asked about is omitted.

From context, we understand it as something like:

• そのことを係員に聞きました
• 分からないことを係員に聞きました

Japanese often leaves out information that is obvious from context.

So the sentence does not need to say exactly what was asked, because the listener already knows:

• there was something unclear
• the speaker asked the staff member about it

Japanese very often puts the reason or background before the main action.

So the structure is:

• Reason/background: 分からないことがあったので
• Main action: その場で係員に聞きました

This is very normal in Japanese. In English, we can also do this:

• Because there was something I didn’t understand, I asked the staff member on the spot.

So the Japanese word order may feel different at first, but it is a standard pattern.

The subject is omitted, but it is understood from context.

Most naturally, it is:

• I

So the full sense is:

• I had something I didn’t understand, so I asked a staff member on the spot.

Japanese often omits:

• I
• you
• he/she
• other obvious information

As long as the context makes it clear, leaving the subject out sounds natural.

It is mildly polite and very natural.

Why:

• The sentence ends with 聞きました, which is the polite past form.
• The earlier parts use plain forms like 分からない, あった, which is normal before the final verb.

This mixture is standard Japanese:

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

So this sounds suitable for:

• speaking politely
• writing a simple explanation
• answering a question about what happened

A more casual version might be:

• 分からないことがあったから、その場で係員に聞いた。

But the original sentence is perfectly natural and polite.