هذه الحركة سريعة.

Because الحركة is a feminine singular noun in Arabic, so it takes the feminine singular demonstrative هذه (this).

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

So:

• هذا الكتاب = this book
• هذه الحركة = this movement

Arabic demonstratives must agree with the noun in gender and number.

Yes. In Arabic, grammatical gender is not the same as biological sex.

A noun can be grammatically feminine even if it refers to a thing, idea, or action. حركة is simply a feminine noun by grammar.

A common clue is the ending ـة (called tāʾ marbūṭa), which often marks feminine nouns:

• حركة
• سيارة
• لغة

So الحركة is grammatically feminine, and that affects the words that agree with it.

Because adjectives in Arabic agree with the noun they describe in gender and number.

Since الحركة is feminine singular, the adjective must also be feminine singular:

• masculine: سريع
• feminine: سريعة

So:

• القطار سريع = the train is fast
• الحركة سريعة = the movement is fast

Here, سريعة matches الحركة.

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

So:

• هذه الحركة سريعة literally looks like this movement fast
• but it means this movement is fast

This is completely normal in Arabic.

If you want was, you usually use كان:

• كانت هذه الحركة سريعة = this movement was fast

Yes, the whole phrase is definite, but in Arabic it is normal for a noun after a demonstrative like هذا / هذه to also take الـ.

So standard Arabic says:

• هذه الحركة

not usually just:

• هذه حركة in the same meaning

This is a very common pattern in MSA:

• هذا الكتاب
• هذه السيارة
• هذا الرجل

So even though English says this movement without the, Arabic normally uses هذه الحركة.

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

There is an important difference:

• هذه الحركة سريعة = This movement is fast.
• هذه الحركة السريعة = This fast movement.

In the first sentence, سريعة tells you something about الحركة, so it stays indefinite.

In the second phrase, السريعة is directly modifying الحركة inside the noun phrase, so it becomes definite and takes الـ.

This is a nominal sentence: a sentence that does not begin with a verb.

It has two main parts:

• هذه الحركة = the topic / subject
• سريعة = the information being said about it

In traditional grammar, these are often called:

• مبتدأ
• خبر

You do not need those terms to understand the sentence, but it helps to know that Arabic often builds simple present-tense statements this way.

A fully vowelled version would normally be:

هٰذِهِ الْحَرَكَةُ سَرِيعَةٌ

The important endings are:

• الحركةُ = nominative
• سريعةٌ = nominative, indefinite

In everyday Arabic writing, these short vowels are usually omitted, so you normally see:

هذه الحركة سريعة

The final ة is called tāʾ marbūṭa.

In pause, it is usually pronounced like -a / -ah:

• حركة → ḥaraka
• سريعة → sarīʿa

But when grammatical endings are pronounced in careful reading, the t sound appears:

• الحركةُ → al-ḥarakatu
• سريعةٌ → sarīʿatun

So learners often hear both patterns depending on how formal or careful the pronunciation is.

Then both the demonstrative and the adjective would change to masculine forms.

Compare:

• هذه الحركة سريعة = feminine
• هذا التغيير سريع = masculine

So the pattern is:

• feminine: هذه ... سريعة
• masculine: هذا ... سريع

This is a good example of how agreement works across the sentence.