هذه الفكرة غريبة.

Because الفكرة is a feminine noun in Arabic, and the demonstrative has to match it.

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

So:

• هذا كتاب = this book
• هذه فكرة = this idea

Since فكرة is feminine, Arabic uses هذه.

A very common clue is the ending ة (taa marbuuTa), which often marks a noun as feminine.

So:

• فكرة = idea
• ends in ة
• therefore it is treated as feminine

Not every feminine noun ends this way, and not every noun ending pattern works exactly the same, but this is one of the most useful early clues for learners.

Because adjectives in Arabic usually agree with the noun they describe in:

• gender (masculine/feminine)
• number
• definiteness in some structures

Here, غريبة refers to الفكرة, which is feminine, so the adjective must also be feminine.

Compare:

• كتاب غريب = a strange book
• فكرة غريبة = a strange idea

So the feminine ending on غريبة matches the feminine noun الفكرة.

In Arabic, the present tense of to be is usually not stated in simple sentences like this.

So:

• هذه الفكرة غريبة literally looks like this idea strange
• but it means This idea is strange

This is very normal in Arabic. In the present tense, Arabic often leaves out is / am / are.

If you wanted a past meaning, Arabic would use a verb, for example:

• كانت هذه الفكرة غريبة = This idea was strange

Because in this sentence, غريبة is a predicate adjective, not an adjective directly attached to the noun inside one noun phrase.

There is an important difference:

1. هذه الفكرة غريبة = This idea is strange

   • غريبة is the predicate: it tells us something about the subject.
   • It is normally indefinite, so no الـ.

2. هذه الفكرة الغريبة = this strange idea

   • الغريبة is now an adjective directly modifying الفكرة
   • so it matches the noun in definiteness and takes الـ

So:

• الفكرة غريبة = the idea is strange
• الفكرة الغريبة = the strange idea

That is a very important distinction in Arabic.

Because هذه الفكرة is the normal MSA way to say this idea.

In standard Arabic, demonstratives like هذا / هذه commonly come before the noun:

• هذا الكتاب = this book
• هذه الفكرة = this idea

You may also hear post-noun demonstratives in some contexts or dialects, but for Modern Standard Arabic, هذه الفكرة is the standard pattern learners should use first.

For a beginner, the easiest way to understand it is to treat هذه الفكرة as the subject phrase: this idea.

Then:

• هذه الفكرة = the subject
• غريبة = the predicate adjective

So the sentence structure is basically:

• this idea
  • strange
• meaning: This idea is strange

In more advanced grammar analysis, Arabic grammarians may describe the internal structure in a more technical way, but for learning and understanding the sentence, thinking of هذه الفكرة as a single noun phrase is perfectly helpful.

In MSA, هذه is pronounced haadhihi.

A rough breakdown:

• ها = haa
• ذِ = dhi
• final هِ = hi

So: haadhihi

Learners sometimes shorten or vary it based on dialect pronunciation, but in careful Modern Standard Arabic, haadhihi is the expected pronunciation.

A common careful pronunciation is:

haadhihi al-fikratu ghariibatun

In normal pause, many speakers simplify the endings, so you may hear something closer to:

haadhihi al-fikra ghariiba

Useful notes:

• الفكرة is often heard as al-fikra in pause
• غريبة is often heard as ghariiba in pause
• in fully vocalized grammar-oriented reading, nominative endings appear:
  • الفكرةُ
  • غريبةٌ

Both are useful to recognize.

The ending ة (taa marbuuTa) usually sounds like:

• -a / -ah when stopping
• -t when the word carries a case ending or is followed in connected grammatical pronunciation

So in pause:

• الفكرة ≈ al-fikra
• غريبة ≈ ghariiba

In full grammatical reading:

• الفكرةُ = al-fikratu
• غريبةٌ = ghariibatun

This is why learners sometimes see ة but do not always hear a clear t sound.

Because the sentence means The/this idea is strange, not the strange idea.

In Arabic nominal sentences:

• the subject can be definite
• the predicate is often indefinite

So here:

• الفكرة is definite because of الـ
• غريبة is indefinite because it is the predicate

This is a common pattern:

• الكتاب جديد = the book is new
• المدينة كبيرة = the city is big
• هذه الفكرة غريبة = this idea is strange

If both were definite, the structure would usually mean something different.

Yes, but the meaning changes slightly.

• هذه الفكرة غريبة = This idea is strange
• هذه فكرة غريبة = This is a strange idea

In the second sentence, فكرة غريبة is indefinite: a strange idea.

So the presence or absence of الـ on فكرة changes the meaning:

• الفكرة = the idea
• فكرة = an idea

That is a very useful contrast to notice.

A masculine version would use:

• هذا instead of هذه
• a masculine noun
• a masculine adjective

For example:

• هذا الكتاب غريب = This book is strange

Compare with your sentence:

• هذه الفكرة غريبة = This idea is strange
• هذا الكتاب غريب = This book is strange

This is a good way to practice gender agreement.

You would normally use تلك for a feminine noun:

• تلك الفكرة غريبة = That idea is strange

So the contrast is:

• هذه الفكرة غريبة = this idea is strange
• تلك الفكرة غريبة = that idea is strange

Since فكرة is feminine, the demonstrative must stay feminine in both cases.

Normally, when an adjective directly modifies a noun, it comes after the noun in Arabic.

So:

• فكرة غريبة = a strange idea
• not غريبة فكرة

In your sentence, غريبة comes after هذه الفكرة because it is the predicate of the sentence:

• هذه الفكرة غريبة = This idea is strange

So whether it is an adjective inside a noun phrase or a predicate adjective in a sentence, Arabic still places it after the noun it relates to.

In fully vocalized formal Arabic, it is:

هٰذِهِ الْفِكْرَةُ غَرِيبَةٌ

The endings are nominative because this is a simple nominal sentence:

• هٰذِهِ = demonstrative, fixed form
• الْفِكْرَةُ = nominative
• غَرِيبَةٌ = nominative predicate

In everyday spoken reading or in pause, those endings are often not pronounced clearly, but it is useful to know them if you are studying grammar or reading fully vowelled text.