tomodati ga kireina utagoe de utaikiru to, ookina hakusyu ga okotta.

が marks 友達 as the subject of 歌い切る: the friend is the one who finishes singing.

Using は would make 友達 the topic instead, which would slightly change the feel:

• 友達が = focusing on the friend as the subject
• 友達は = as for my/the friend...

In this sentence, が works well because the sentence is narrating an event: the friend sang, and then applause happened.

きれい is a な-adjective, not an い-adjective, even though it ends in -い.

So:

• before a noun: きれいな歌声
• at the end of a sentence: 歌声がきれいだ

That is why you need な before 歌声.

歌声 means singing voice or the sound of someone singing.

It is different from:

• 声 = voice in general
• 歌 = a song
• 歌声 = the voice as it is heard in singing

So きれいな歌声 is not just a beautiful voice in a general sense, but specifically a beautiful singing voice.

Here, で shows the means or manner.

So きれいな歌声で歌い切る means something like:

• to sing all the way through with a beautiful singing voice
• to finish singing in a beautiful voice

This is not the location-use of で. It is closer to with / by means of / in in English.

歌い切る is a compound verb:

• 歌う = to sing
• 切る = to cut, but in compound verbs it often means to do something completely, all the way through, or to the end

So 歌い切る means:

• to sing to the end
• to finish singing completely
• to get through the song

Using 歌う would just mean to sing.
Using 歌い切る adds a sense of completion.

Before と in this kind of sentence, Japanese often uses the plain non-past form, even when talking about a past event.

So:

• 歌い切ると、大きな拍手が起った。

means:

• When/As soon as the friend finished singing, big applause broke out.

This pattern is very common in Japanese narration:

• ドアを開けると、猫がいた。 = When I opened the door, there was a cat.

The past time is shown by the main verb 起った, not by making 歌い切る past.

No. Here と does not mean and.

It means something like:

• when
• as soon as
• once

It often gives the feeling that the second event happens naturally, immediately, or as a result of the first one.

So:

• 歌い切ると、大きな拍手が起った。 = When/As soon as the friend finished singing, there was a big round of applause.

This と is different from the quotation particle と and different from the and particle used to connect nouns.

Yes, but the nuance would change.

• 歌い切ると
  = when/as soon as; sounds like a natural sequence of events

• 歌い切ったら
  = when/after finishing; a little more neutral and flexible

• 歌い切ったとき
  = at the time when the friend finished singing; focuses more on the time point itself

In this sentence, と is good because it strongly suggests: the friend finished singing, and immediately applause broke out.

Because 拍手 is the subject of the second verb, 起った.

So the sentence has two clauses:

1. 友達が ... 歌い切る

   • subject: 友達

2. 大きな拍手が起った

   • subject: 大きな拍手

This is completely normal in Japanese.
Each clause can have its own subject.

Yes, literally it is big applause, but in natural English it means:

• loud applause
• great applause
• a big round of applause

Japanese often uses 大きい / 大きな with things like sounds, reactions, or effects in a way that English may translate less literally.

So 大きな拍手 is a very natural Japanese expression.

Japanese often uses 起こる for events that occur, happen, or break out.

So 拍手が起こる means:

• applause breaks out
• applause arises
• there is applause

It sounds natural in Japanese, even if English would not normally say applause occurred in everyday speech.

A very natural translation would be:

• A big round of applause broke out.

Yes, 起こった is the more common modern spelling.

• dictionary form: 起こる
• past form: 起こった

The spelling 起った is a more abbreviated kanji-heavy way of writing the same word.
You may see it in literature, older texts, or stylistic writing, but for learners, 起こった is usually the form to remember first.