Üç bisküvi yedim, dördüncüsünü de istiyorum.

In Turkish, nouns usually stay singular after a number.

• üç bisküvi = three biscuits
• not normally üç bisküviler

This is different from English, where we say three biscuits with a plural noun. In Turkish, the number already shows plurality, so the noun does not need a plural ending.

Yedim means I ate.

It comes from the verb yemek = to eat.

Breakdown:

• ye- = verb stem
• -di = past tense
• -m = I

So:

• yedim = I ate

The form looks slightly irregular because yemek becomes ye- in many conjugations.

Because the sentence describes two different times:

• Üç bisküvi yedim = something that already happened
• dördüncüsünü de istiyorum = what I want now

So the timeline is:

1. I ate three biscuits.
2. Now I want the fourth one too.

This is completely natural in Turkish.

İstiyorum is the normal way to say I want in a current, real situation.

• istiyorum = I want / I am wanting now
• isterim = more like I do want / I usually want / I would want, depending on context

In modern Turkish, the -yor form is very commonly used for present meaning, even with verbs like istemek. So here istiyorum is the most natural choice.

Dördüncü means fourth.

It comes from:

• dört = four
• -üncü = ordinal suffix, making fourth

So:

• dört = four
• dördüncü = fourth

This is an ordinal number, like first, second, third, fourth in English.

Because Turkish is turning fourth into the fourth one and then marking it as the object of the verb.

Step by step:

• dördüncü = fourth
• dördüncüsü = the fourth one
• dördüncüsünü = the fourth one as a definite direct object

So in this sentence, dördüncüsünü means the fourth one.

This is very common in Turkish: an adjective or ordinal can take extra endings and act like a noun.

They are grammatical endings.

1. -sü

This is the 3rd person possessive form used in structures like the X one.

• dördüncü = fourth
• dördüncüsü = the fourth one

The s appears because the word already ends in a vowel.

2. -nü

This is the accusative ending, showing that the word is a specific direct object.

• dördüncüsünü istiyorum = I want the fourth one

The n is a buffer consonant that appears before the accusative here.

So the whole form is:

• dört
  • ordinal suffix → dördüncü
• • -sü → dördüncüsü
• • accusative -nü → dördüncüsünü

This is a very important Turkish pattern.

üç bisküvi yedim

Here the object is indefinite / non-specific: just three biscuits in general.
So there is no accusative ending.

dördüncüsünü de istiyorum

Here the object is specific: the fourth one.
So Turkish uses the accusative.

Compare:

• bisküvi yedim = I ate biscuits / a biscuit
• bisküviyi yedim = I ate the biscuit

So the accusative often signals that the object is definite or specific.

Here de means too / also.

So:

• dördüncüsünü de istiyorum = I want the fourth one too

It adds the idea of also.

Depending on context, de can sometimes feel like even or as well, but here too / also is the best understanding.

Because this de is a separate word, not a suffix.

Turkish has:

• de/da = too, also → written separately
• -de/-da = in, at, on → written attached

So:

• ben de = me too
• evde = at home

In your sentence:

• dördüncüsünü de = the fourth one too

So it must be written separately.

No.

They look similar, but they are different things:

Separate de

Means too / also

• Ben de geldim. = I came too.

Attached -de

Means in / at / on

• Evdeyim. = I am at home.

In dördüncüsünü de istiyorum, it is the first kind: too / also.

It is understood and therefore omitted.

The first clause already mentions bisküvi, so the second clause does not need to repeat it. Turkish often avoids repetition this way.

So instead of saying:

• dördüncü bisküviyi de istiyorum

the sentence says:

• dördüncüsünü de istiyorum

That means roughly I want the fourth one too.

This sounds natural and efficient.

Yes, you could.

• dördüncü bisküviyi de istiyorum = I want the fourth biscuit too
• dördüncüsünü de istiyorum = I want the fourth one too

The second version is more natural if the noun has already been mentioned. It avoids repeating bisküvi.

So both are possible, but dördüncüsünü is smoother in this context.

Not really. Dördüncüsünü points to the fourth one in a sequence.

So the idea is:

• first biscuit
• second biscuit
• third biscuit
• now the fourth one

If you wanted to say something more like I want another biscuit or I want one more biscuit, Turkish would more naturally use something like:

• Bir tane daha istiyorum. = I want one more.

So dördüncüsünü is specifically the fourth one, not just another one.