هذه كلمة جديدة.

Because كلمة is a feminine singular noun in Arabic, and the demonstrative this has to agree with it.

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

So with كلمة, you say هذه.

A useful comparison:

• هذه كلمة جديدة = This is a new word
• هذا كتاب جديد = This is a new book

Arabic has grammatical gender, not just real-life biological gender.

So a noun can be grammatically masculine or feminine even if the thing itself has no natural sex. كلمة is simply a feminine noun in Arabic.

One clue is its ending:

• كلمة
• the final ة is called tāʾ marbūṭa
• this ending very often marks feminine nouns

So كلمة is feminine because that is its grammatical form, not because a word is literally female.

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

So Arabic often says something that is literally closer to:

• this a new word

But in normal English, we translate it as:

• This is a new word

This kind of sentence is called a nominal sentence in Arabic.

If you wanted the past or future, Arabic would use a verb, but in the simple present it is normally omitted.

Arabic has no separate indefinite article like English a/an.

Instead, a noun is usually understood as indefinite because:

• it does not have الـ on it
• and in fully vocalized Arabic it may take tanwīn (the -n ending)

So:

• كلمة = a word / word
• الكلمة = the word

That is why كلمة جديدة means a new word, even though there is no separate word for a.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• كلمة جديدة = literally word new
• natural English: new word

This is the normal Arabic pattern:

• noun first
• adjective second

Examples:

• كتاب جديد = a new book
• سيارة كبيرة = a big car

Because the adjective has to agree with the noun it describes.

Since كلمة is:

• feminine
• singular
• indefinite

the adjective must also be:

• feminine
• singular
• indefinite

So:

• كلمة جديدة = correct
• كلمة جديد = incorrect

This agreement is very important in Arabic. Adjectives usually match the noun in:

• gender
• number
• definiteness
• case

Because that would be a different structure.

In هذه كلمة جديدة, هذه stands on its own as this, and كلمة جديدة is the rest of the statement. So the meaning is:

• This is a new word

But when you say هذه الكلمة الجديدة, you are making a demonstrative noun phrase with a definite noun:

• هذه الكلمة الجديدة
• roughly this new word

So the sentence you were given uses the indefinite noun كلمة because the idea is This is a new word, not just this new word as a phrase.

That final ة is called tāʾ marbūṭa.

It often marks a noun or adjective as feminine.

In this sentence:

• كلمة ends in ة, so it is feminine
• جديدة also ends in ة, showing the adjective is feminine too

In careful grammar/pronunciation, this ending can sound like t when case endings are pronounced:

• كلمةٌ → kalimatun
• جديدةٌ → jadīdatun

But in pause or simpler pronunciation, it is often heard more like -a:

• kalima
• jadīda

A common learner-friendly pronunciation is:

• هذه = hādhihi
• كلمة = kalima
• جديدة = jadīda

So the whole sentence is often read as:

• hādhihi kalima jadīda

In fully inflected formal Arabic, you may hear:

• hādhihi kalimatun jadīdatun

A couple of pronunciation notes:

• ذ in هذه sounds like the th in this
• ج in Modern Standard Arabic is usually like j in judge

Those are case endings plus tanwīn.

In fully vocalized formal Arabic:

• كَلِمَةٌ
• جَدِيدَةٌ

the -un shows that the words are:

• indefinite
• and in the nominative case here

Why nominative? Because كلمة جديدة is the predicate part of a simple nominal sentence.

For beginners, the important thing is:

• the basic written sentence usually appears without these vowel marks
• many speakers do not pronounce full case endings in everyday reading/speech

So you will often just see:

• هذه كلمة جديدة

without the endings written out.

You would change the demonstrative and the adjective to the masculine form.

Example:

• هذا كتاب جديد
• This is a new book

Compare:

• هذه كلمة جديدة → feminine singular
• هذا كتاب جديد → masculine singular

So the pattern stays the same, but the forms have to agree with the noun’s gender.