Lütfen anahtarı komodinin üstüne koy.

Because anahtarı has the accusative ending. In Turkish, a specific/definite direct object usually takes this ending.

So:

• anahtar = a key / key
• anahtarı = the key

In this sentence, the speaker means a particular key, not just any key, so anahtarı is used.

This is the genitive form. Turkish often expresses ideas like on the table, under the bed, inside the box with a structure that is more literally like:

• the table’s top
• the bed’s underside
• the box’s inside

So:

• komodin = nightstand / bedside table
• komodinin üstü = the top of the nightstand

That is why komodin becomes komodinin.

Turkish usually does not use prepositions the same way English does. Instead, it often uses a noun-like location word such as:

• üst = top
• alt = bottom / underneath
• iç = inside
• ön = front
• arka = back

So komodinin üstüne literally means something like to the top of the nightstand, which is how Turkish expresses onto/on top of the nightstand.

Because the verb koymak involves movement toward a place.

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

So:

• komodinin üstüne koy = put it onto the nightstand
• anahtar komodinin üstünde = the key is on the nightstand

A simple way to remember it:

• -e / -a often shows direction toward
• -de / -da often shows location in/on/at

Üstüne can be broken down like this:

• üst = top
• -ü = its top
• -ne = to/onto it

So üstüne literally means onto its top.

In komodinin üstüne, the full idea is onto the top of the nightstand.

Koy is the 2nd person singular imperative of koymak (to put / to place).

So it means:

• Put it.
• Place it.

It is said to one person.

Related forms:

• koy = put! (to one person)
• koyun = put! (to more than one person, or more politely to one person)

Yes. Lütfen means please, so it softens the imperative.

Without lütfen, Anahtarı komodinin üstüne koy. is more direct. With lütfen, it becomes more polite and natural in many everyday situations.

Still, it is grammatically a command. If you want to sound even softer or more formal, you could say:

• Lütfen anahtarı komodinin üstüne koyar mısınız?
• Anahtarı komodinin üstüne koyabilir misiniz?

Yes and no.

The normal Turkish pattern is:

• object + place + verb

So:

• Lütfen anahtarı komodinin üstüne koy.

This is very natural because Turkish usually puts the verb at the end.

Some parts can move for emphasis, for example:

• Anahtarı lütfen komodinin üstüne koy.

But koy will usually stay at the end.

Yes, very often you can.

• komodinin üstüne koy
• komodinin üzerine koy

Both can mean put it on top of the nightstand.

In many situations, they are almost interchangeable. Üstüne is very common in everyday speech. Üzerine can sound a little more neutral or slightly more formal, depending on context.

Komodin usually means a nightstand or bedside table. It can sometimes refer to a small cabinet-like piece of furniture, but in this sentence the most natural meaning is nightstand.

So komodinin üstüne means onto the top of the nightstand.

Yes. The command form would usually change.

To one person, informal:

• Lütfen anahtarı komodinin üstüne koy.

To more than one person, or more politely to one person:

• Lütfen anahtarı komodinin üstüne koyun.

Even more polite:

• Lütfen anahtarı komodinin üstüne koyar mısınız?

So the main change is in the verb form.