apuri ga kidousita.

Yes. アプリ is the shortened everyday form of アプリケーション, which comes from English application.

In modern Japanese, アプリ is the normal word for an app, especially on phones, tablets, and computers. The full form アプリケーション sounds more formal or technical.

Here, が marks アプリ as the subject of the sentence.

So the basic structure is:

• アプリ = app
• が = subject marker
• 起動した = started up / launched

The idea is that the app is the thing that underwent the action or change of state. In other words, the app is what started up.

起動した is the plain past form of 起動する.

Breakdown:

• 起動 = startup, activation
• する = to do
• 起動する = to start up, to launch
• 起動した = started up / launched

This is a very common noun + する pattern in Japanese. These are often called する-verbs.

Japanese past tense often shows that a change has been completed.

So 起動した does not only mean something happened in the distant past. It can also mean:

• the app started up
• the app has started up
• the app is now up and running

In context, it often focuses on the fact that the startup is complete.

Yes, but the meaning changes.

• アプリが起動した = the app started up
• アプリを起動した = someone started the app

With が, the app is the subject and the sentence describes the app coming on.

With を, the app is the object, and the subject is an omitted person or agent, such as I, you, or the system.

So:

• アプリが起動した focuses on what happened to the app
• アプリを起動した focuses on who started it, even if that person is not said explicitly

Both are possible, but the nuance changes.

• アプリが起動した is a neutral statement of fact: the app started up
• アプリは起動した makes アプリ the topic: as for the app, it started up

Using は can sound contrastive depending on context. For example, it may suggest something like:

• the app started up, but something else did not
• at least the app started up

So in an out-of-the-blue statement about an event, が is very natural.

Yes, it is somewhat technical or computer-related.

起動する is commonly used for:

• apps
• programs
• computers
• systems
• devices

It means something like start up, boot up, or launch.

Compared with other verbs:

• 起動する = technical, system-like startup
• 開く = open
• 立ち上がる = come up, boot up, start running

For software, 起動する is very natural.

No. Standard Japanese normally does not use spaces between words.

The normal written sentence is:

アプリが起動した。

Spaces are often added in teaching materials to make word boundaries easier for learners to see.

Not necessarily.

Japanese often describes events from the point of view of the result, without saying who caused them. So アプリが起動した simply tells you that the app started up.

It does not strongly tell you whether:

• the user tapped it
• the system launched it automatically
• it started for some other reason

If you want to clearly say that someone launched it, アプリを起動した is better.

Yes. 起動した is plain style, not polite style.

The polite version would be:

アプリが起動しました。

Both mean the same basic thing, but:

• 起動した = plain, casual, neutral
• 起動しました = polite

You choose based on the situation and who you are speaking to.