kanozyo no hannou ha hayakatta.

の links two nouns. Here it shows possession or association, so 彼女の反応 means her reaction.

You can think of it as working a lot like of or 's in English:

• 彼女の反応 = her reaction
• literally, something like the reaction of her

It does not mean that she is actively doing something here; it just connects 彼女 and 反応.

は marks the topic of the sentence.

So in 彼女の反応は速かった, the topic is her reaction, and the sentence says something about that topic: it was quick.

A natural breakdown is:

• 彼女の反応 = her reaction
• は = as for...
• 速かった = was quick / was fast

So the whole sentence is basically:

• As for her reaction, it was quick.

That sounds awkward in English, but it shows how the Japanese structure works.

速い is the non-past form of an い-adjective, meaning fast/quick.

To make it past tense, you change:

• 速い → 速かった

So:

• 速い = is fast / is quick
• 速かった = was fast / was quick

This sentence is talking about a reaction that already happened, so the past form 速かった is used.

For most い-adjectives, you replace the final い with かった.

Examples:

• 高い → 高かった = was expensive / was tall
• 寒い → 寒かった = was cold
• 速い → 速かった = was fast

So here:

• dictionary form: 速い
• past form: 速かった

Yes, both are read はやい, but they usually mean different things:

• 早い = early
• 速い = fast, quick, speedy

A reaction is about speed, not earliness, so 速い is the right kanji here.

Examples:

• 朝早い = early in the morning
• 足が速い = runs fast
• 反応が速い = reacts quickly / has a quick reaction

By itself, 彼女 can mean:

• she
• her
• sometimes girlfriend, depending on context

In this sentence, because it is followed by の, it functions like her:

• 彼女の反応 = her reaction

So here it is not naturally understood as girlfriend’s reaction unless the larger context strongly suggests that.

Yes, very often in Japanese, if the person is already clear from context, you could simply say:

• 反応は速かった。

That would mean something like:

• The reaction was quick
• Her reaction was quick
• His reaction was quick

Japanese often leaves out information that English usually states directly, as long as the listener can figure it out from context.

That is also possible, but it is a slightly different structure.

Compare:

• 彼女の反応は速かった。
• 彼女は反応が速かった。

The first one focuses on her reaction as a noun phrase:
Her reaction was quick.

The second one makes she the topic and says, roughly:
As for her, her reaction was quick.

Both can be natural, but 彼女の反応は速かった is a very direct way to say her reaction was quick.

If you said:

• 彼女の反応は速かった。

you are treating her reaction as the topic and commenting on it.

If you said:

• 彼女の反応が速かった。

that sounds more like you are identifying her reaction as the thing that was quick, often with a more specific or contrastive nuance depending on context.

A simple way to think about it:

• は = topic, as for...
• が = subject focus, the one that...

In many beginner contexts, both may translate similarly, but the nuance is different.

反応 means reaction or response.

It is a noun, so in this sentence it works exactly like the English noun reaction:

• 彼女の反応 = her reaction

You may also see related expressions like:

• 反応する = to react
• 反応がいい = to respond well / to be responsive
• 反応が遅い = reaction is slow

So this sentence is describing the speed of her reaction.

It is pronounced:

• かのじょ の はんのう は はやかった

A few notes:

• 彼女 = かのじょ
• 反応 = はんのう
• 速かった = はやかった

Even though 早かった and 速かった use different kanji, they are both pronounced はやかった.

Correct: normal Japanese writing usually does not use spaces.

So it would normally be written as:

• 彼女の反応は速かった。

Spaces are often added in learning materials to help beginners see the parts more clearly:

• 彼女 / の / 反応 / は / 速かった

That is just a teaching aid, not standard native writing.

Yes. The literal structure is Her reaction was quick, but in natural English that can often be expressed as:

• She reacted quickly

So the Japanese sentence focuses on the noun reaction, but the natural English translation might use a verb instead.

Both are good ways to understand it:

• Her reaction was quick.
• She reacted quickly.

It is in the plain style because it ends with 速かった, not 速かったです.

Compare:

• 彼女の反応は速かった。 = plain
• 彼女の反応は速かったです。 = polite

The plain form is very common in casual speech, narration, and written Japanese.

So this sentence is not rude; it is just less polite than the です form.