kami wo hanbun ni oru mae ni, haha ha zyougi wo tukaimasita.

Because 紙 is the direct object of 折る (to fold).

In 紙を半分に折る, the thing being folded is paper, so it takes を.

• 紙を折る = to fold paper
• 本を読む = to read a book
• パンを食べる = to eat bread

Even though 紙を半分に折る comes before 前に, it is still a complete verb phrase, and 紙 is still the object of 折る.

Here, に marks the resulting state or form.

So 半分に折る means to fold it into half or to fold it in half.

This に is not a location marker here. It shows what the paper becomes as a result of the action.

Compare:

• 紙を半分に折る = fold the paper in half
• 氷が水になる = ice becomes water
• 部屋をきれいにする = make the room clean

So in this sentence, 半分に tells you the shape/state the paper ends up in.

Because when Japanese says before doing X, it normally uses:

• dictionary form + 前に

So:

• 折る前に = before folding
• 食べる前に = before eating
• 行く前に = before going

This is a very common grammar pattern.

By contrast:

• 食べた後で = after eating
• 行った後で = after going

So 折る前に is correct because the action of folding has not happened yet at that point.

前 means before or in front of, and に marks the time point here.

In this sentence, 折る前に means before folding it.

The structure is:

• [action] + 前に = before [action]

So the whole first part:

• 紙を半分に折る前に

means:

• before folding the paper in half

Then the main action happens:

• 母は定規を使いました = Mother used a ruler.

So the full sentence means that the ruler was used before the paper was folded.

In this grammar pattern, 前に is the normal form.

• 食べる前に
• 寝る前に
• 出かける前に

The に helps mark the time when the main action happens relative to the other action.

In casual speech, you may sometimes hear 前 without に, especially in shorter expressions, but for learners, verb + 前に is the standard pattern to remember.

は marks 母 as the topic of the sentence.

So 母は means something like:

• As for mother, ...
• Mother, ...

In this sentence, the speaker is telling us what the mother did:

• 母は定規を使いました = Mother used a ruler.

If you used が, it would sound more like you are specifically identifying mother as the one who did it, often with a contrast or new-information feeling.

Very roughly:

• 母は定規を使いました。 = As for mother, she used a ruler.
• 母が定規を使いました。 = Mother is the one who used a ruler.

For a simple neutral statement like this, は is very natural.

Because 使う (to use) usually takes the thing being used as a direct object.

So:

• 定規を使う = use a ruler
• ペンを使う = use a pen
• 日本語を使う = use Japanese

That is why the sentence says:

• 母は定規を使いました

not 定規で使いました.

The ruler is the thing being used, so it takes を.

It breaks down like this:

• 紙を半分に折る前に = before folding the paper in half
• 母は = mother
• 定規を使いました = used a ruler

So the structure is:

• [Before doing X], [someone] did Y.

More literally:

• Before folding the paper in half, mother used a ruler.

Japanese often places the time/background information first, then gives the main action afterward.

By itself, 紙を半分に折る前に does not explicitly say who folds it.

In Japanese, if the subject is obvious from context, it is often omitted.

In this sentence, many readers will naturally assume the mother is the one doing the folding too, because 母は is the only stated person in the sentence. But grammatically, the subject of 折る is left unstated.

So the sentence does not force an English-style explicit subject like:

• Before she folded the paper in half...

Japanese often leaves that kind of thing to context.

Because the sentence is describing something that happened.

• 使う = use
• 使いました = used

The polite ending -ました makes it polite, and the past form -ました shows the action already happened.

So:

• 使います = uses / will use
• 使いました = used

Since the sentence is talking about what happened before the paper was folded, the past tense is appropriate.

Because 折る前に is a grammar pattern, not the main tense-bearing part of the sentence.

The main event of the sentence is:

• 母は定規を使いました

That is where the sentence tense is shown.

The phrase 折る前に just means before folding and does not need to be in past tense.

This is similar to English in a way:

• Before folding the paper, mother used a ruler.

English also does not say before folded there.

Yes, that is close to the literal structure.

A more natural English translation would usually be:

• Before folding the paper in half, Mother used a ruler.

The Japanese 半分に折る is often best translated as fold in half rather than fold into half, even though に does show a result state.

So the Japanese grammar is doing exactly what you think, but the most natural English wording is a little different.

Usually, 母 refers to one's own mother when speaking to someone else.

Japanese often uses:

• 母（はは） = my mother
• お母さん（おかあさん） = mother / someone else's mother / mom

So in many contexts, 母は would naturally mean my mother.

However, in isolated example sentences for grammar study, it may simply be used as a neutral mother unless the context matters.

Here are the readings:

• 紙 = かみ
• 半分 = はんぶん
• 折る = おる
• 前 = まえ
• 母 = はは
• 定規 = じょうぎ
• 使いました = つかいました

So the full sentence is read:

• かみ を はんぶん に おる まえ に、はは は じょうぎ を つかいました。

It can be attached to both.

With a verb:

• 寝る前に = before sleeping
• 行く前に = before going

With a noun:

• 食事の前に = before the meal
• テストの前に = before the test

Notice that with a noun, you usually add の:

• noun + の + 前に

But with a verb, you use the dictionary form directly:

• verb dictionary form + 前に

So this sentence uses the verb pattern:

• 折る前に

No, it is not grammatically required, but it is very common and helpful.

The comma shows a pause after the introductory time phrase:

• 紙を半分に折る前に、母は定規を使いました。

Without the comma, the sentence is still correct:

• 紙を半分に折る前に母は定規を使いました。

Japanese punctuation is often more flexible than English punctuation, but the comma makes the sentence easier to read.