kaisya ga apuri wo kaihatusita.

が marks 会社 as the grammatical subject of the verb 開発した.

So in this sentence:

• 会社が = the company (as the one doing the action)
• アプリを = the app (as the thing being acted on)
• 開発した = developed

If you used は instead, the nuance would change. は introduces the topic, while が identifies the subject more directly.

Compare:

• 会社がアプリを開発した。
  = The company developed the app.
  Focus on the company as the doer.

• 会社はアプリを開発した。
  = As for the company, it developed the app.
  This sounds more like you are talking about the company as a topic, perhaps in contrast with something else.

Both can be natural, but が is a very straightforward choice when simply stating who did the action.

を marks the direct object of the verb.

In this sentence:

• アプリを = the app as the thing that was developed

The verb 開発する means to develop, and the thing being developed is marked with を.

So:

• 会社が = the company did it
• アプリを = the app received the action
• 開発した = developed

This is one of the most basic sentence patterns in Japanese:

A が B を Verb

= A does Verb to B

Because Japanese normally uses subject–object–verb word order, unlike English, which usually uses subject–verb–object.

English:

• The company developed the app.

Japanese:

• 会社 が アプリ を 開発した。
• literally: Company subject app object developed

The verb usually comes at the end of the clause in Japanese. That is one of the biggest structural differences from English.

開発した is the plain past affirmative form of 開発する.

Breakdown:

• 開発する = to develop
• 開発した = developed / did develop

This is a suru-verb:

• noun: 開発 = development
• verb: 開発する = to do development / to develop

Then it changes like this:

• 開発する = develop / will develop
• 開発した = developed
• 開発しない = do not develop / will not develop
• 開発しなかった = did not develop

した is the plain form, while しました is the polite form.

So:

• 開発した。 = plain style
• 開発しました。 = polite style

The sentence you were given is in plain style, which is common in:

• dictionary examples
• casual writing
• conversations with friends
• textbook grammar examples

If you wanted to make the whole sentence polite, you would say:

• 会社がアプリを開発しました。

Both mean the same thing, but the level of politeness is different.

Yes. アプリ is a very common Japanese shortened form of アプリケーション, which comes from English application.

In modern Japanese, アプリ usually means:

• phone app
• software app
• application program

It is written in katakana because it is a loanword.

Japanese uses different writing systems for different kinds of words.

Here:

• 会社 is a native/common Japanese word usually written in kanji
• アプリ is a loanword, so it is written in katakana

This mixture is completely normal in Japanese. A single sentence can contain:

• kanji for core vocabulary
• hiragana for particles and endings
• katakana for foreign loanwords

So this sentence is a very typical example of mixed Japanese writing.

No. In normal Japanese writing, this sentence would usually be written without spaces:

• 会社がアプリを開発した。

Spaces are often added in learning materials to help beginners see the parts more clearly.

So the spaced version is for study, not standard writing.

Japanese nouns usually do not show singular/plural the way English nouns do.

So アプリ could mean:

• app
• apps

The exact meaning depends on context.

The same is true for 会社:

• company
• companies

In this sentence, English translation may use the company and the app, but Japanese itself does not force that distinction here.

Japanese does not have articles like English a, an, and the.

So:

• 会社 can mean a company, the company, or just company
• アプリ can mean an app, the app, or just app

Context tells you which English article makes the most sense.

That is why translating Japanese into natural English often requires adding words that are not explicitly present in the Japanese sentence.

Yes, if it is already clear from context.

Japanese often omits information that speakers already understand. So if everyone already knows who developed the app, you could simply say:

• アプリを開発した。

This could mean:

• (Someone) developed the app.
• (We/they/the company) developed the app.

Japanese frequently leaves out the subject when it is obvious.

It mainly means to develop, especially in contexts like:

• software
• technology
• products
• land/resources
• new methods

In this sentence, because the object is アプリ, the meaning is naturally to develop an app.

So 開発した sounds like:

• created and built it
• worked on it as a product
• developed it technically

It is a very common business and tech word.

The non-past form is:

• 会社がアプリを開発する。

This can mean:

• The company develops apps.
• The company will develop an app.

Japanese non-past can refer to:

• present habitual action
• future action

Context decides which is intended.

By contrast:

• 会社がアプリを開発した。 = The company developed the app.

It is pronounced:

• かいはつした
• kaihatsu shita

Breakdown:

• 開発 = かいはつ
• した = shita

So the whole sentence is:

• かいしゃ が アプリ を かいはつした。
• kaisha ga apuri o kaihatsu shita

A small pronunciation note: を is pronounced basically like お in modern Japanese, even though it is written を as a particle.

Yes. It is grammatically complete and natural.

It is a simple statement with:

• a subject: 会社が
• an object: アプリを
• a verb: 開発した

So it works perfectly well as a standalone sentence meaning that the company developed the app.

In real-life conversation, Japanese might omit some parts if they are obvious, but as a clear full example sentence, this is very natural.