هذه القصة مثيرة.

Because القصة is a feminine singular noun.

In Modern Standard Arabic, the demonstrative this must agree with the noun:

• هذا = this for masculine singular
• هذه = this for feminine singular

So:

• هذا الكتاب = this book
• هذه القصة = this story

Since قصة is feminine, هذه is the correct form.

A very common clue is the ending ة , called taa marbuuTa.

Many Arabic nouns ending in ة are feminine, including:

• قصة = story
• سيارة = car
• مدينة = city

So when you see القصة, you can often expect feminine agreement in the rest of the sentence.

In Arabic, when a demonstrative like هذا or هذه comes before a noun, that noun is usually definite, so it normally takes ال.

So Arabic says:

• هذه القصة literally something like
• this the-story

That is just the normal Arabic pattern. In English, this story does not use the, but Arabic does.

In Arabic, the present-tense verb to be is usually not written or said in simple sentences like this.

So:

• هذه القصة مثيرة means
• This story is exciting

There is no separate word for is in the present tense here.

But in the past or future, Arabic does use words for was / will be, such as:

• كانت القصة مثيرة = The story was exciting
• ستكون القصة مثيرة = The story will be exciting

Because it describes القصة, which is feminine.

Arabic adjectives must agree with the noun they describe in gender and number. So:

• مثير = exciting, masculine singular
• مثيرة = exciting, feminine singular

Since القصة is feminine singular, the correct form is مثيرة.

Because here مثيرة is the predicate of the sentence, not an adjective inside a noun phrase.

There is an important difference:

• القصة المثيرة = the exciting story

  • Here المثيرة is a regular adjective modifying القصة
  • It takes ال because it matches the definite noun

• القصة مثيرة = the story is exciting

  • Here مثيرة is the predicate
  • It is normally indefinite, so it does not take ال

So in your sentence, مثيرة means is exciting, not the exciting.

It is a nominal sentence in Arabic.

You can think of it as:

• هذه القصة = the topic / subject, this story
• مثيرة = the comment / predicate, exciting

So the whole structure is:

• subject
  • predicate

This is one of the most common sentence patterns in Arabic.

Usually, when you pause at the word, ة sounds like -a or -ah.

So you would normally hear:

• القصة ≈ al-qiSSa
• مثيرة ≈ muthiira

But in fully pronounced formal Arabic, when grammatical endings are added, the ة is pronounced as t:

• القصةُ = al-qiSSatu
• مثيرةٌ = muthiiratun

So the same letter can sound different depending on how fully the sentence is being read.

Yes. In normal Arabic writing, short vowels and case endings are usually omitted.

So the sentence is commonly written:

• هذه القصة مثيرة

But if fully vocalized, it would be:

• هٰذِهِ القِصَّةُ مُثِيرَةٌ

Those endings show grammar:

• القصةُ is nominative
• مثيرةٌ is also nominative as the predicate

Learners usually first meet the unvocalized version, because that is how most real Arabic text is written.

Yes, that is a very helpful way to think about it.

هذه القصة works together as a single phrase meaning this story. Then مثيرة tells you something about that phrase.

So mentally you can divide the sentence like this:

• هذه القصة | مثيرة
• this story | exciting

That makes the sentence easier to parse.