Ben çantayı yanına koyuyorum.

In Turkish, the verb ending already shows the subject. Koyuyorum means I am putting, so ben is not required.

The sentence could simply be Çantayı yanına koyuyorum.

Adding ben usually gives a little more emphasis, contrast, or clarity, as in I’m the one putting the bag next to him/her/you.

Because it is the direct object and it is specific/definite.

Turkish often marks a specific direct object with the accusative ending -(y)ı / -(y)i / -(y)u / -(y)ü. Here:

çanta + -yı → çantayı

The y is a buffer consonant added because çanta ends in a vowel.

So çantayı means the bag or that specific bag as the thing being put somewhere.

Yanına literally means to his/her/its side or to your side, depending on context. In natural English, it usually means next to him/her/it/you.

It comes from yan, which means side. Turkish often uses words like side, inside, top, front, and so on with possessive and case endings to express meanings that English often expresses with prepositions.

So instead of one simple word for next to, Turkish often builds it this way.

Yes. The form yanına is ambiguous by itself.

It can come from: yanın + a = to your side or yanı + na = to his/her/its side

So without more context, you cannot know for sure who it refers to.

If Turkish wants to make it explicit, it can say: senin yanına = next to you / to your side onun yanına = next to him/her / to his/her side

Because Turkish often does not use prepositions the same way English does.

English says: next to him to your side

Turkish often expresses this with a noun plus endings: yan = side then a possessive ending then a case ending

So yanına is a built form meaning something like to his/her/your side. This is a very common Turkish pattern.

It is the present continuous, meaning an action that is happening now or is in progress.

So koyuyorum means I am putting or I’m placing.

Depending on context, the Turkish present continuous can also sometimes refer to a near-future action, but in a basic sentence like this, the most natural reading is I am putting.

It breaks down like this:

koy- = put, place
-uyor- = present continuous marker here
-um = I

So:

koy-uyor-um → koyuyorum

The vowel in the continuous suffix changes because of vowel harmony. The dictionary form is often shown as -iyor, but in real words it changes shape: -ıyor / -iyor / -uyor / -üyor

Here it becomes -uyor, so we get koyuyorum.

Because Turkish normally puts the verb at the end in neutral word order.

So a typical structure is:

Ben = subject
çantayı = object
yanına = destination/location-related phrase
koyuyorum = verb

This final-verb pattern is one of the most important word-order habits in Turkish.

Yes, Turkish word order is flexible, but changing it usually changes emphasis rather than the basic meaning.

For example: Çantayı yanına koyuyorum. = neutral, with ben omitted
Ben çantayı yanına koyuyorum. = slightly more emphasis on I
Çantayı ben yanına koyuyorum. = stronger contrast, like I’m the one putting the bag next to him/her/you

So the meaning stays similar, but the focus shifts.

This is a very important difference.

yanına means to his/her/your side or next to him/her/you as a destination. It shows movement toward that place.

yanında means at his/her/your side or beside him/her/you as a location. It shows where something already is.

So:

Çantayı yanına koyuyorum. = I am putting the bag next to him/her/you.
Çanta yanında. = The bag is next to him/her/you.

In short: -a / -e often shows movement toward -da / -de often shows location at

Koymak usually means to put or to place something somewhere deliberately.

That makes it very natural in a sentence about placing a bag in a certain spot.

Bırakmak can also sometimes mean leave or put down, but it often has a slightly different feel, such as leaving something somewhere, letting go, or dropping something off.

So in this sentence, koymak is the straightforward verb for put/place.