هذه الطائرة كبيرة.

Because الطائرة is a feminine singular noun, and the demonstrative this has to match it.

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

So:

• هذا كتاب = this is a book / this book
• هذه طائرة = this is an airplane / this airplane

In your sentence, the noun is feminine, so هذه is the correct form.

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

So in الطائرة:

• the base word is طائرة
• it ends in ـة
• that usually means it is grammatically feminine

Not every feminine noun ends this way, and not every word ending this way behaves identically, but it is one of the most useful patterns for beginners.

Because adjectives in Arabic must agree with the noun they describe.

Since الطائرة is:

• feminine
• singular

the adjective big must also be:

• feminine
• singular

So:

• كبير = big (masculine singular)
• كبيرة = big (feminine singular)

That is why the sentence uses كبيرة, not كبير.

In Arabic, adjectives normally come after the noun they describe.

So Arabic says:

• الطائرة الكبيرة = literally the airplane the-big

where English says:

• the big airplane

This is a normal word-order difference between Arabic and English.

In your sentence, كبيرة comes after الطائرة for that reason.

In the present tense, Arabic usually does not use a separate word for is / am / are in simple sentences like this.

So:

• هذه الطائرة كبيرة literally looks like this airplane big
• but it means This airplane is big

This is completely normal in Arabic.

If you wanted to say was, then Arabic would use a verb, for example:

• كانت هذه الطائرة كبيرة = This airplane was big

This is a very important point.

In هذه الطائرة كبيرة, the word كبيرة is not just an adjective inside a noun phrase. It is the predicate of the sentence: big.

So the structure is:

• هذه الطائرة = this airplane / this plane
• كبيرة = is big

That is why كبيرة is indefinite here.

Compare these:

• هذه الطائرة كبيرة = This airplane is big
• هذه الطائرة الكبيرة = This big airplane

In the second sentence, الكبيرة is a true adjective modifying الطائرة, so it becomes definite too.

They mean different things:

• هذه الطائرة كبيرة = This airplane is big
• هذه الطائرة الكبيرة = This big airplane

In the first one:

• كبيرة is the predicate of the sentence

In the second one:

• الكبيرة is an adjective attached to الطائرة

This distinction is one of the most important things to notice in Arabic sentence structure.

Usually, الطائرة is pronounced aṭ-ṭāʾirah, not al-ṭāʾirah.

That is because ط is a sun letter. With sun letters, the l sound of الـ is not pronounced; instead, the next consonant is doubled.

So:

• written: الطائرة
• pronounced: aṭ-ṭāʾirah

This is a pronunciation rule only; the spelling still keeps الـ.

Here هذه is a demonstrative, meaning this.

It points to the noun:

• هذه الطائرة = this airplane

It is not an adjective like big. It is a different kind of word.

In Arabic grammar, demonstratives like هذا and هذه often come before the noun they point to.

In fully vowelled, formal grammar, case endings can appear, for example:

• هذه الطائرةُ كبيرةٌ

But in most normal Arabic writing, short vowels and case endings are omitted, so you usually see:

• هذه الطائرة كبيرة

This is standard and normal. Learners often meet the unvowelled version first in everyday texts.

The final ـة (tāʾ marbūṭa) is usually pronounced like -a or -ah when pausing.

So you will commonly hear:

• ṭāʾirah
• kabīrah

In fully inflected formal Arabic, if the word takes an ending, the t sound can appear, for example in connected speech or grammatical forms such as:

• طائرةٌ = ṭāʾiratun
• كبيرةٌ = kabīratun

But for a beginner, it is usually enough to remember:

• when stopping, it often sounds like -a / -ah

Not exactly as written.

هذه الطائرة كبيرة most directly means:

• This airplane is big

If you want This is a big airplane, Arabic would more naturally be:

• هذه طائرة كبيرة

Notice the difference:

• هذه الطائرة كبيرة = This airplane is big
• هذه طائرة كبيرة = This is a big airplane

The presence or absence of الـ makes an important difference.