Battaniyeyi koltuğun üstüne koydum.

Because it is the direct object of the verb koydum and it is a specific/definite thing.

In Turkish, a specific direct object usually takes the accusative ending.
So:

• battaniye = blanket
• battaniyeyi = the blanket, as the object

There is also a buffer letter y between the noun and the ending because the noun ends in a vowel:

• battaniye + i would be awkward
• so Turkish uses y: battaniye + yi = battaniyeyi

If the object were non-specific, Turkish might leave off the accusative:

• Battaniye koydum = I put a blanket
• Battaniyeyi koydum = I put the blanket

This happens for two reasons:

1. It takes the genitive ending, because it is part of the structure koltuğun üstü = the couch’s top / the top of the couch
2. The final k in koltuk softens to ğ before a vowel-initial ending

So:

• koltuk = couch
• koltuk + un = expected genitive form
• but k softens: koltuğun

This is a very common sound change in Turkish:

• kitap → kitabı
• ağaç → ağacı
• koltuk → koltuğun

Because koltuğun üstüne is more specific. It means something like onto the top/surface of the couch.

Compare:

• koltuğa koydum = I put it on/to the couch
• koltuğun üstüne koydum = I put it onto the top of the couch

Turkish often uses body-part/location nouns like üst (top, upper side) in these expressions.

So the structure is:

• koltuğun = of the couch
• üstü = its top
• üstüne = onto its top

This is very natural Turkish.

Üstüne is built from üst plus endings.

Step by step:

• üst = top, upper side
• üstü = its top
• üstüne = onto its top

The ending -e / -a here is the dative, which often shows movement toward something.

So:

• üstünde = on top of it / on it
• üstüne = onto it / onto the top of it

That is why koltuğun üstüne koydum expresses movement toward that surface: the blanket was placed there.

Yes, this is a common Turkish pattern called a genitive-possessive construction.

In English, we say:

• the top of the couch

In Turkish, that becomes:

• koltuğ-un üst-ü

Literally:

• couch-GEN top-POSS

So the first noun gets the genitive ending, and the second noun gets a possessive ending.

This pattern is extremely common:

• evin kapısı = the door of the house
• masanın ayağı = the leg of the table
• koltuğun üstü = the top of the couch

Then Turkish adds another case ending to the second word if needed:

• koltuğun üstü = the top of the couch
• koltuğun üstüne = onto the top of the couch
• koltuğun üstünde = on the top of the couch

Koydum comes from the verb koymak = to put, to place.

It breaks down like this:

• koy- = verb stem
• -du- = past tense
• -m = I

So koydum means I put.

The past tense ending appears in different forms because of vowel harmony and consonant harmony:

• -dı
• -di
• -du
• -dü
• or sometimes -tı, -ti, -tu, -tü

Here it becomes -dum, giving koydum.

It is not said separately, because Turkish verbs usually already show the subject.

In koydum, the ending -m tells you the subject is I.

So Turkish often drops subject pronouns when they are already clear:

• Ben koydum = I put it
• Koydum = I put it

Both are possible, but ben is often omitted unless you want emphasis or contrast.

Because Turkish is normally an SOV language: Subject – Object – Verb.

A very typical order is:

• object
• place/direction
• verb

So:

• Battaniyeyi = the blanket
• koltuğun üstüne = onto the couch
• koydum = I put

That said, Turkish word order is somewhat flexible. You can move parts around for emphasis, but the neutral/basic order usually puts the verb at the end.

Yes, but the emphasis changes.

The neutral version is:

• Battaniyeyi koltuğun üstüne koydum.

Other orders are possible, such as:

• Koltuğun üstüne battaniyeyi koydum.
• Battaniyeyi koydum koltuğun üstüne.
  This is less neutral and more marked in style or emphasis.

Because Turkish uses endings like -yi and -e, the grammatical roles are still clear even when word order changes. But for learners, the original sentence is the safest and most natural basic pattern.

Because the sentence describes movement to a place, not just location at a place.

• üstüne = onto
• üstünde = on it / on top of it

The verb koymak involves placing something somewhere, so Turkish uses the form that shows direction toward that place.

Compare:

• Battaniye koltuğun üstünde. = The blanket is on the couch.
• Battaniyeyi koltuğun üstüne koydum. = I put the blanket onto the couch.

So -e here matches the idea of motion or placement.

Yes, koltuğun üzerine koydum is also possible and natural.

In many contexts, üstüne and üzerine are very close in meaning:

• koltuğun üstüne koydum
• koltuğun üzerine koydum

Both can mean I put it on/onto the couch.

Very roughly:

• üstüne often feels a bit more everyday and concrete
• üzerine can feel slightly more formal or broad, depending on context

For a learner, the important thing is that üstüne in this sentence is completely normal and idiomatic.

Yes, there are a few very common Turkish sound patterns here:

1. Buffer consonant y

   • battaniye + i → battaniyeyi
   • Turkish often inserts y between two vowels

2. Consonant softening

   • koltuk → koltuğun
   • final k becomes ğ before a vowel-initial ending

3. Vowel harmony

   • endings change to match earlier vowels
   • that is why you get forms like -yi, -un, -e, -du

These patterns appear constantly in Turkish, so this sentence is a very good example of how endings work in real usage.