Imperator pacis causa nuntium mittit.

Imperator is nominative singular. It is the subject of the sentence: the person who is doing the action of mittit.

Depending on context, imperator can mean commander, general, or later emperor, but grammatically here it is simply the subject.

Because of the expression pacis causa.

When causa means for the sake of or because of, Latin regularly uses:

genitive + causa

So:

• pax = peace
• pacis = of peace

That is why Latin says pacis causa, literally for the sake of peace.

In this expression, causa behaves almost like a postposition rather than a normal noun.

So Latin usually says:

• pacis causa = for the sake of peace
• pecuniae causa = for the sake of money
• belli causa = for the sake of war

This word order is standard in this idiom.

If you said causa pacis, that would more naturally sound like the cause of peace rather than for the sake of peace.

Nuntium is accusative singular.

It is the direct object of mittit, because it is the thing being sent.

A very common pattern in Latin is:

subject + accusative object + verb

So here:

• imperator = subject
• nuntium = direct object
• mittit = verb

It can be confusing, because the form nuntium can represent two different words:

• nuntius, nuntii = messenger
  
  • accusative singular: nuntium
• nuntium, nuntii = message / news
  
  • accusative singular: nuntium

So the form by itself is ambiguous. You tell which one it is from the context or from the translation already given.

Mittit is:

• 3rd person singular
• present tense
• active voice
• indicative mood

It comes from mittere, meaning to send.

The ending -t tells you it means he/she/it sends.

So imperator ... mittit means the commander sends.

Because Latin usually does not need an expressed subject pronoun when the verb ending already shows who is acting.

The form mittit already means he/she/it sends, so Latin can simply say:

Imperator ... mittit

instead of adding a separate word for he.

Latin has no articles like English the and a/an.

So imperator can mean:

• the commander
• a commander

and nuntium can mean:

• the messenger
• a messenger
• the message
• a message

You decide from context which English article fits best.

Not as fixed as in English.

Because Latin uses case endings, the job of each word is usually clear even if the order changes. So the sentence could be rearranged in different ways without changing the basic meaning.

For example, Latin could also say things like:

• Nuntium imperator pacis causa mittit
• Imperator nuntium mittit pacis causa

However, pacis causa usually stays together as a unit.

Latin word order often affects emphasis more than basic meaning.

Yes. It is a very common and important pattern.

You will often see:

• genitive + causa
• genitive + gratia

Both can mean for the sake of.

Examples:

• honoris causa = for the sake of honor
• auxilii causa = for the sake of help
• pacis gratia = for the sake of peace

So this sentence is useful because it teaches a standard Latin construction, not just a one-time phrase.