sore wa yuukanna koudou deshita.

topic particle

Breakdown of sore wa yuukanna koudou deshita.

• です: to be
• それ: that
• 行動こうどう: an action or behavior

Questions & Answers about sore wa yuukanna koudou deshita.

What does は do in this sentence?

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.

Why is it それは and not それが?

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.