kare no koudou ha hayakatta.

The sentence breaks down like this:

• 彼 = he / him
• の = marks possession or connection
• 行動 = action, behavior, conduct
• は = topic marker
• 早かった = was quick / was early

So the structure is:

[彼の行動] は [早かった]
= As for his actions/behavior, they were quick

A very literal gloss would be:

his action/behavior + topic + was quick

の links one noun to another. Here, it shows that 行動 belongs to or is associated with 彼.

So:

• 彼の行動 = his actions / his behavior

This is very similar to the English 's or of:

• 彼の本 = his book
• 先生の車 = the teacher's car

In this sentence, 彼の is simply modifying 行動.

は marks the topic of the sentence. It tells you what the sentence is about.

So 彼の行動は means:

As for his behavior/actions...

Then the sentence comments on that topic with 早かった.

If you changed は to が, the nuance would shift a bit:

• 彼の行動は早かった = as for his actions, they were quick
• 彼の行動が早かった = his actions were the thing that was quick

In many contexts both are possible, but は often sounds more like you are presenting his behavior as the topic and then describing it.

行動 means action, behavior, or conduct.

It is broader than a single physical movement. It often refers to how someone acts in a situation, what they do, or how quickly they take action.

So in this sentence, 行動 is not just one body motion like lifting a hand. It is more like:

• what he did
• how he acted
• how quickly he took action

Depending on context, English translations might be:

• His actions were quick
• He acted quickly
• He was quick to act

早かった is the past tense of the い-adjective 早い.

The pattern is:

• 早い = quick / early
• remove い
• add かった
• 早かった = was quick / was early

This is the normal past affirmative pattern for い-adjectives:

• 高い → 高かった = was expensive / was high
• 寒い → 寒かった = was cold
• 早い → 早かった = was quick / was early

So 早かった is not a verb. It is an adjective in past form.

This is a very common question because both 早い and 速い are read はやい.

The difference is:

• 早い = early in time, quick to do something, prompt
• 速い = fast in speed

In 彼の行動は早かった, the idea is usually that he was quick to act or prompt, so 早い is the natural choice.

Examples:

• 行動が早い = quick to act, prompt
• 足が速い = runs fast
• 車が速い = the car is fast

So this sentence is more about timing and promptness than pure physical speed.

Yes, that is a very natural English translation in many contexts.

Even though the Japanese literally talks about his behavior/actions, English often sounds more natural if you translate it more freely:

• He acted quickly
• He was quick to act
• His response was quick

A very literal translation like His behavior was quick is understandable, but it may sound a little unnatural in English depending on context.

So the best English version depends on the situation.

It is grammatical, but in real Japanese, 彼 is often used less than English he.

Japanese commonly does one of these instead:

• omit the subject if it is clear
• use the person's name
• use a title or role, such as 部長 or 先生

So while 彼の行動は早かった is correct, in natural conversation you might also hear something like:

• 田中さんの行動は早かった
• 行動は早かった

depending on context.

So learners should know that 彼 is fine, but Japanese does not rely on pronouns as heavily as English does.

Yes, that is possible, but the focus is slightly different.

Compare:

• 彼の行動は早かった
  Focuses on his actions/behavior

• 彼は行動が早かった
  Focuses on him, and then says that his actions were quick

In English, both may end up sounding similar. In Japanese, the choice depends on what you want as the main topic.

If you are specifically discussing his behavior, 彼の行動は早かった is very straightforward.

Because Japanese adjectives can function as predicates by themselves.

In English, you usually need a verb such as was:

• His actions were quick

In Japanese, 早かった already carries the predicate meaning, so no extra was-type verb is needed here.

That is normal for Japanese い-adjectives:

• この本は高い = This book is expensive
• 昨日は寒かった = It was cold yesterday
• 彼の行動は早かった = His actions were quick

So the sentence is complete even without a separate main verb like する.

Those spaces are usually just for learners.

Normal Japanese is written without spaces:

彼の行動は早かった。

Textbooks and learning materials sometimes add spaces to make the parts easier to see:

彼　の　行動　は　早かった。

So the spaced version is helpful for study, but the standard written form does not use those spaces.