sensei ga kaesita touan wo mitara, bunpou no matigai ga ikutuka atta.

Because Japanese uses a relative clause before the noun it modifies.

So 先生が返した答案 means:

• the answer sheet that the teacher returned

In English, the noun usually comes first and the description comes after it:

• the answer sheet that the teacher returned

In Japanese, it is the other way around:

• 先生が返した
  • 答案

This is a very common pattern:

• 私が昨日買った本 = the book I bought yesterday
• 友だちが作ったケーキ = the cake my friend made

Here, 先生 is the subject of 返した.

• 先生が返した答案 = the answer sheet that the teacher returned

Inside that mini-clause, the teacher is the one doing the action of returning, so が marks the subject.

This が only belongs to the relative clause:

• 先生が返した

It does not mark the subject of the whole sentence.

When a clause modifies a noun in Japanese, there is normally no extra word or particle between them.

So:

• 先生が返した答案 not
• 先生が返したの答案

The clause directly attaches to the noun.

This is just how Japanese relative clauses work:

• 昨日見た映画 = the movie I saw yesterday
• 母が作った料理 = the food my mother made

Because 答案 is the direct object of 見たら.

The structure is:

• 先生が返した答案 = the answer sheet the teacher returned
• を見たら = when/after I looked at it

So the whole first part means:

• when I looked at the answer sheet that the teacher returned

Even though 答案 is modified by the clause 先生が返した, it is still the thing being looked at, so it takes を.

Here it means when I looked or after I looked.

The たら form can express:

• a condition: if
• a time sequence: when/after

In this sentence, the second clause tells you something was discovered as a result of looking:

• 文法の間違いがいくつかあった = there were several grammar mistakes

So 見たら is best understood as a temporal/discovery meaning:

• when I looked
• after looking

A natural interpretation is:

• When I looked at the answer sheet the teacher returned, there were several grammar mistakes.

Because they belong to different clauses.

1. 先生が返した

   • 先生 is the subject of 返した

2. 文法の間違いがいくつかあった

   • 文法の間違い is the subject of あった

So the sentence has two parts:

• 先生が返した答案を見たら
• 文法の間違いがいくつかあった

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

Because の is connecting two nouns.

• 文法 = grammar
• 間違い = mistake

So 文法の間違い means:

• grammar mistakes
• mistakes in grammar

The particle の often works like:

• of
• or a noun modifier

Compare:

• 日本の車 = Japanese cars / cars of Japan
• 英語の先生 = English teacher / teacher of English

Here, 文法の間違い means mistakes related to grammar.

いくつか means several or some.

In this sentence:

• 文法の間違いがいくつかあった = there were several grammar mistakes

This placement is very natural with ある and いる:

• 本が何冊かある = there are several books
• 問題がいくつかあった = there were several problems

You could also see:

• いくつかの文法の間違い which means several grammar mistakes

But with existence verbs like ある, Japanese often uses:

• noun + が + quantity + ある

Because 間違い is inanimate.

Japanese uses:

• ある for inanimate things
• いる for animate beings, especially people and animals

So:

• 間違いがあった = there was a mistake
• 学生がいた = there was a student

Since grammar mistakes are not living things, ある / あった is the correct choice.

The subject is omitted because Japanese often leaves out information that is understood from context.

So 見たら here most likely means:

• when I looked or possibly
• when we looked depending on context

Japanese does this all the time:

• 食べた = I ate / he ate / she ate / they ate
• 行きます = I will go / he will go / etc.

The listener usually figures it out from the situation. In this sentence, the natural assumption is that the speaker looked at their own returned answer sheet.

Not exactly.

答案 refers to an answer sheet, exam paper, or written answers, and it sounds a bit more formal or academic.

So:

• 答案 = answer sheet / exam paper
• テスト = test, quiz, exam
• 紙 = paper

In this sentence, 答案 is a good word because the teacher is returning a student’s written work, and the speaker is checking it afterward.

Usually 先生が返した答案 is the more natural choice here.

In a relative clause, が is commonly used to mark the subject of the action:

• 先生が返した = the teacher returned

Using は inside a relative clause is possible in some contexts, but it often adds a contrastive or topical nuance and is less neutral here.

So for a straightforward description, が is the standard choice:

• 先生が返した答案 = the answer sheet that the teacher returned