Learn Turkish: Dünya, büyük bir gezegendir. - The Earth is a large planet.

The suffix -dir is the factual copula. In Turkish, it is used to state objective facts, absolute truths, and encyclopedic information. Because "The Earth is a large planet" is a scientific fact, -dir is used here to state it formally.

How do we know to use -dir instead of -tır, -dur, or -dür?

This suffix adapts to the word it attaches to using both vowel harmony and consonant harmony. The last vowel in gezegen is 'e', so 4-way vowel harmony requires an 'i'. Because gezegen ends in 'n' (which is a voiced consonant, not a hard one like p, ç, t, k), the suffix begins with a 'd'. This gives us -dir.

Why is the word order büyük bir gezegendir instead of bir büyük gezegendir?

In English, we say "a large planet", putting the article "a" first. In Turkish, when a noun has both a descriptive adjective and the indefinite article bir, the word bir must go directly between them. The rule is always Adjective + bir + Noun.

Why is there a comma immediately after Dünya?

In Turkish, commas are frequently used right after the subject of a sentence to clearly separate it from the predicate. This visually marks Dünya as the subject being described, making the sentence easier to parse.