haha ha kireina binsen wo hanbun ni otte, huutou ni iremasita.

は marks 母 as the topic of the sentence: As for my mother, ....
That is very natural when you are simply telling what she did.

If you used 母が, it would put more focus on mother as the subject, often with a nuance like it was mother who... or when introducing her as new information.

So in this sentence, 母は is the normal, neutral choice.

きれい is a na-adjective, so when it directly modifies a noun, it needs な:

• きれいな便せん = beautiful / pretty letter paper

Compare:

• 高い山 = a high mountain
• 静かな部屋 = a quiet room

Even though きれい ends in い, it is not an i-adjective. That is why you say きれいな, not きれいい.

便せん means letter paper or stationery for writing a letter. It refers to the sheet of paper you write the letter on, not the envelope.

So in this sentence:

• 便せん = the paper
• 封筒 = the envelope

The action is: fold the letter paper, then put it into the envelope.

を marks the direct object of the verb. Here, the thing being folded is the letter paper:

• 便せんを折って = folded the letter paper

Later, the same letter paper is understood as the thing being put into the envelope:

• 封筒に入れました = put it into the envelope

Japanese often does not repeat the object if it is already clear from context.

半分に means in half or more literally into halves / into a half-state.

With verbs like 折る or 切る, に often marks the resulting state:

• 紙を半分に折る = fold the paper in half
• ケーキを半分に切る = cut the cake in half

So 便せんを半分に折って means folded the letter paper in half.

折って is the te-form of 折る. Here it connects two actions in sequence:

1. folded it in half
2. put it into the envelope

So:

• 折って、封筒に入れました = folded it and put it into the envelope

Only the final verb, 入れました, carries the tense/politeness for the whole sequence. This is very common in Japanese.

In this sentence, the て-form links actions and usually means and then.

So:

• 折って、封筒に入れました
  = folded it, and then put it into the envelope

The exact nuance can vary in Japanese, but here it is a simple sequence of actions.

Because 封筒 is the destination/container, not the direct object.

• に marks where something goes: into the envelope
• the omitted direct object is the letter paper

So the structure is:

• 便せんを 半分に折って、封筒に 入れました
  = folded the letter paper in half and put it into the envelope

If you said 封筒を入れました, that would mean put in the envelope as the object, which is not what is happening here.

Because 入れる is the transitive verb to put something in, while 入る is the intransitive verb to enter / go in.

Here, the mother is actively putting the letter paper into the envelope, so Japanese uses the transitive verb:

• 入れる = put in
• 入る = go in / enter

Examples:

• 手紙を封筒に入れる = put the letter into the envelope
• 手紙が封筒に入る = the letter fits/goes into the envelope

入れました is the polite past form of 入れる.

So it tells you:

• polite style: ます
• past tense: ました

That means the whole sentence is politely describing something that already happened.

Yes, in a sentence like this, 母 usually means my mother when the speaker is talking about their own mother.

In Japanese, family terms often change depending on whether you are talking about:

• your own family to outsiders
• someone else’s family
• directly addressing a family member

So:

• 母 = my mother
• お母さん = mother / someone else’s mother / Mom, depending on context

In this sentence, 母 is the natural way to refer to one’s own mother in narration.

The comma is mainly there to make the sentence easier to read. It shows a small pause between the two actions:

• fold it in half,
• put it in the envelope

Japanese punctuation is often flexible, so the sentence could still be understood without the comma. It is not a special grammar point by itself here.

Yes. The direct object of 入れました is not repeated, but it is understood to be the letter paper.

So the full idea is:

• Mother folded the beautiful letter paper in half and put it into the envelope.

Japanese very often leaves out words that are obvious from context, especially subjects and objects.