Bu büyük bir gezegendir.

Here is the breakdown:

• Bu = this
• büyük = big / large
• bir = a / an (or one, depending on context)
• gezegen = planet
• -dir = a copular ending, roughly is, often with a slightly formal, definite, or explanatory tone

So the sentence means This is a big planet.

In Turkish, the verb to be is often not expressed as a separate word in simple present-tense sentences.

So instead of using a separate word like English is, Turkish usually just puts the predicate after the subject:

• Bu büyük bir gezegen. = This is a big planet.

The ending -dir can be added:

• Bu büyük bir gezegendir.

That makes the sentence sound more formal, more definite, or more like a statement of fact.

-dir is the third-person copular ending. In a sentence like this, it corresponds loosely to is, but it is not always necessary.

Compare:

• Bu büyük bir gezegen. = neutral, everyday
• Bu büyük bir gezegendir. = more formal, more assertive, more explanatory

You often hear -dir in:

• definitions
• written language
• formal speech
• scientific or factual statements

So this sentence sounds a bit like a textbook statement: This is a large planet.

Yes. Absolutely.

• Bu büyük bir gezegen.
  is a perfectly correct and very natural sentence.

In everyday Turkish, speakers often omit -dir in this kind of sentence. The version with -dir is not wrong at all; it just sounds more formal or emphatic.

This is a very common question for English speakers.

In Turkish, when a noun has an adjective, the usual order is:

adjective + bir + noun

So:

• büyük bir gezegen = a big planet
• güzel bir ev = a beautiful house
• küçük bir araba = a small car

This is different from English, where a comes before the adjective:

• a big planet

So büyük bir gezegen is the normal Turkish pattern.

Here, bir means a.

Turkish bir can mean either:

• one
• a / an

In this sentence, it is understood as the indefinite article:

• büyük bir gezegen = a big planet

If you strongly stressed bir, it could sometimes feel more like one, but in this sentence the normal reading is simply a big planet.

Because Turkish is an agglutinative language, which means endings are attached directly to the word.

So:

• gezegen
  • dir = gezegendir

This is normal in Turkish. Grammatical endings are usually added to the end of words rather than written separately.

The literal order is:

• Bu = this
• büyük bir gezegen = a big planet
• -dir = is

So Turkish is structured more like:

This a big planet is

But in natural English, we say:

This is a big planet

Turkish often puts the predicate before the copular ending, and the whole sentence follows Turkish word order rules rather than English ones.

Yes.

Bu can stand alone and mean this. It can also be used before a noun:

• bu gezegen = this planet

But in your sentence, bu stands on its own as the subject:

• Bu büyük bir gezegendir. = This is a big planet.

That is completely normal.

Yes, slightly.

The version with -dir sounds more formal, careful, or factual than the version without it.

So:

• Bu büyük bir gezegen. = natural everyday speech
• Bu büyük bir gezegendir. = more formal, textbook-like, explanatory

Neither is wrong. They just have different tones.

Yes, and it means something very close, but the structure is different.

• Bu büyük bir gezegendir. = This is a big planet.
• Bu gezegen büyüktür. = This planet is big.

The first sentence identifies this as a big planet.
The second sentence talks about this planet and says that it is big.

So the meaning is similar, but not exactly the same.

The base word is:

• gezegen = planet

Then the copular ending is added:

• gezegen + dir = gezegendir

It appears as -dir here because of Turkish vowel harmony and consonant rules. The actual ending can have different forms, such as:

• -dır
• -dir
• -dur
• -dür
• and voiceless variants like -tır, -tir, -tur, -tür

In gezegen, the final vowel is e, so -dir is the correct form here.

In normal conversation, many speakers would more naturally say:

• Bu büyük bir gezegen.

The version with -dir is still correct, but it sounds a bit more like:

• a definition
• a written statement
• a formal explanation
• a scientific observation

So if you are speaking casually, the shorter version is often more natural.

A rough pronunciation guide is:

Boo buy-YOOK beer geh-zeh-GEN-deer

A few helpful notes:

• Bu sounds like boo
• büyük has the Turkish ü, which does not exist in normal English; it is pronounced with rounded lips
• bir is pronounced like beer, but shorter and lighter
• gezegen has hard g sounds, not like English j
• -dir sounds like deer, but shorter and cleaner

If you want to sound more natural, try to keep the vowels clear and pure rather than turning them into English-style diphthongs.