sakki ookina kaminari no oto ga site, sukosi kowakatta desu.

Both 大きい and 大きな can come before a noun, but they are used a little differently.

• 大きい is the regular -い adjective
• 大きな is a special form used only directly before a noun

In this sentence, 大きな雷の音 means a loud/big sound of thunder.

With 音, 大きな音 is very common and natural. It often feels a bit more descriptive than 大きい音, though both are possible in many situations.

雷の音 literally means the sound of thunder.

Breakdown:

• 雷 = thunder, or the thunder/lightning phenomenon in general
• の = links nouns, like of
• 音 = sound

So this part is focusing specifically on the sound, not the flash of lightning.

This is because 音がする is a very common Japanese pattern.

• 音がする = a sound is heard / there is a sound
• においがする = there is a smell
• 声がする = a voice is heard

Here, する does not mean to do. It means something more like to occur / to be sensed.

So:

• 雷の音がする = thunder can be heard / there is a sound of thunder

That is why が is used with 音 here.

して is the て-form of する. It is being used to connect two parts of the sentence:

• 雷の音がして = there was a sound of thunder, and...
• 少し怖かったです = I was a little scared

So the whole sentence is:

• thunder sounded, and I was a little scared

If the sentence ended after that clause, you would normally say 雷の音がした.

Using して lets the speaker continue to the result or reaction.

This is a very common learner question.

The て-form itself does not show past or non-past by itself. It just connects clauses. The time is understood from context.

Here, the past meaning is clear because:

• さっき means a little while ago
• the second clause is past: 怖かったです

So the whole sentence is naturally understood as talking about something that happened earlier.

Because 怖い is an -い adjective, and -い adjectives make the past like this:

• 怖い → 怖かった
• polite: 怖かったです

You do not say 怖いでした.

More examples:

• 寒い → 寒かったです
• 高い → 高かったです
• 楽しい → 楽しかったです

So 少し怖かったです is the correct polite past form.

Japanese very often leaves out the subject when it is obvious from context.

Here, 少し怖かったです naturally means:

• I was a little scared

The speaker is talking about their own feeling, so 私 is unnecessary unless you want emphasis or contrast.

You could say 私は少し怖かったです, but in normal conversation it is often omitted.

少し means a little or a bit.

So 少し怖かったです means:

• I was a little scared
• It was kind of scary

It softens the statement. The speaker is not saying they were extremely frightened, just somewhat scared.

A few similar words:

• 少し = a little, somewhat
• ちょっと = a bit, a little more conversational
• とても = very

Yes, that is normal in everyday Japanese.

• さっき is casual and common in speech
• です makes the sentence polite

This kind of mix is very natural in conversation. It is not a problem.

If you wanted a more formal version, you could use 先ほど instead of さっき:

• 先ほど、大きな雷の音がして、少し怖かったです。

Yes, it is natural.

This sentence works well and clearly means that the speaker heard a loud thunder sound and felt a bit scared.

A very common alternative would be:

• さっき雷が鳴って、少し怖かったです。

Here:

• 雷が鳴る = thunder rumbles / thunder crashes

That version focuses more directly on the thunder happening.
The original sentence, 雷の音がして, focuses more on hearing the sound.

Both are natural; they just frame the event a little differently.