sore ha yuukanna koudou desita.

は marks それ as the topic of the sentence.

So それは means something like:

• as for that
• that, speaking of it
• that was ...

In this sentence, the speaker is taking それ and saying something about it: it was a brave action.

A very literal breakdown is:

• それ = that
• は = topic marker
• 勇敢な行動 = brave action
• でした = was

So the structure is basically: As for that, it was a brave action.

Both are possible in Japanese, but they do different jobs.

• は marks the topic
• が often marks the subject and can sound more specific or focused

In それは勇敢な行動でした, the sentence sounds like a normal statement about that thing/event.

If you said それが勇敢な行動でした, it could sound more like:

• That was the brave action
• That is the one that was brave
• a contrastive or identifying statement, depending on context

So for a plain descriptive sentence, は is the more natural choice.

Because 勇敢 is a na-adjective, not an i-adjective.

Japanese adjectives are often taught in two main groups:

• i-adjectives: usually end in い, like 高い, 速い
• na-adjectives: take な before a noun, like 静か, 有名, 勇敢

So:

• 勇敢な行動 = brave action

You cannot say 勇敢い行動.

A good way to remember it: when a na-adjective directly modifies a noun, it needs な.

Yes. In Japanese grammar, 勇敢 is a na-adjective.

That means it behaves differently from English adjectives. It does not need to look like an English adjective, and it does not need to end in い.

Compare:

• 静かな町 = a quiet town
• 有名な人 = a famous person
• 勇敢な行動 = a brave action

When a na-adjective comes before a noun, use な.

When it is used at the end of a sentence, it works with だ / です / でした:

• 彼は勇敢だ
• 彼は勇敢です
• 彼は勇敢でした

行動 means action, act, conduct, or behavior, depending on context.

In this sentence, 勇敢な行動 means:

• a brave action
• a courageous act

It is a noun, and 勇敢な modifies it.

So the phrase works as one noun phrase:

• 勇敢な行動 = brave action

This is more specific and natural than just saying something vague like 勇敢でした if the speaker wants to describe what that was.

でした is the polite past form of です.

So:

• です = is / am / are
• でした = was / were

That means the sentence is talking about something in the past:

• それは勇敢な行動です。 = That is a brave action.
• それは勇敢な行動でした。 = That was a brave action.

The sentence is also polite because it uses でした rather than the plain form.

The plain-form version is:

それは勇敢な行動だった。

Here:

• でした = polite past
• だった = plain past

Both mean was, but the tone is different:

• でした = polite, neutral, suitable in many situations
• だった = casual/plain, used with friends, in narration, inner thoughts, etc.

So the meaning stays basically the same, but the level of formality changes.

Japanese usually does not use articles like English a, an, or the.

So 勇敢な行動 could mean:

• a brave action
• the brave action

Which one sounds right depends on context.

That is very normal in Japanese. The listener is expected to understand from the situation what is meant.

So when translating into English, you choose a or the based on context, but the Japanese sentence itself does not need to mark that difference.

The basic order here is very natural:

• それは
  • 勇敢な行動
    • でした

Japanese often puts the main descriptive content before the verb/copula at the end.

You do have some flexibility in Japanese, but not every rearrangement sounds equally natural.

For example, this sentence is standard and clear:

• それは勇敢な行動でした。

You generally would not move でした away from the end.

Also, 勇敢な must stay directly before 行動, because it modifies that noun.

So while Japanese word order can be flexible in some ways, this sentence already has the normal, expected order.

Yes, very often Japanese leaves out topics or subjects when they are clear from context.

So if everyone already knows what is being talked about, you might simply say:

勇敢な行動でした。

That would still mean something like:

• It was a brave action
• That was a brave action

Japanese often omits information that English would usually state explicitly.
But if you want to clearly point to that, then keeping それは is helpful.

Because それ is a pronoun meaning that.

Japanese has two related patterns:

1. Pronouns:

   • これ = this
   • それ = that
   • あれ = that over there

2. Modifiers before nouns:

   • この
     • noun = this ...
   • その
     • noun = that ...
   • あの
     • noun = that ... over there

So:

• それは勇敢な行動でした。 = That was a brave action.
• その行動は勇敢でした。 = That action was brave.

In your sentence, それ stands alone, so the pronoun form is used.

No, normal Japanese writing usually does not use spaces between words.

So it would normally be written as:

それは勇敢な行動でした。

Spaces are often added in beginner materials to make the parts easier to see:

• それ
• は
• 勇敢な
• 行動
• でした

That is just a learning aid, not standard Japanese spelling.