Ben çantayı sandalyenin altına koyuyorum.

That’s right: in Turkish, the subject pronoun is often omitted because the verb ending already shows the person.

• koyuyorum = I am putting
• so ben is not strictly necessary

You could simply say:

• Çantayı sandalyenin altına koyuyorum.

Adding ben can give a little extra emphasis, contrast, or clarity, as in I’m the one putting it there.

Because it is the definite direct object of the sentence.

The verb koymak means to put/place, and the thing being put is the bag. In Turkish, a definite direct object usually takes the accusative case.

• çanta = bag
• çantayı = the bag

The ending here is -(y)ı. The y is a buffer letter added because çanta ends in a vowel.

So:

• çanta = a bag / bag
• çantayı = the bag (as a direct object)

Here it shows that the object is specific/definite.

Compare:

• Çanta koyuyorum. = I am putting a bag / bags.
• Çantayı koyuyorum. = I am putting the bag.

English often marks this difference with a vs the. Turkish often marks it with the accusative ending on the object.

Because alt in this kind of expression behaves like a noun meaning something like the lower part / underside. Turkish often expresses location with a structure like:

• sandalyenin altı = the chair’s underside / the area under the chair

So sandalye takes the genitive ending:

• sandalye = chair
• sandalyenin = of the chair

This is why Turkish says something more literally like:

• the chair’s under-part

rather than using a simple preposition like English under.

Altına can be broken down like this:

• alt = lower part / underneath
• -ı = 3rd person possessive suffix, giving altı = its underside / the underside of it
• -na = dative ending to

So:

• sandalyenin altına = to under the chair = under the chair (with movement toward that location)

This whole phrase is used because the action is moving something to a place.

That n is a buffer consonant.

After a possessed noun form like altı, Turkish often inserts n before another vowel-initial ending such as a case suffix.

So:

• altı
  • -a becomes altına, not altıa

This is a normal and very common pattern in Turkish grammar.

Because altına shows movement toward that place, while altında shows location at/in that place.

• sandalyenin altına koyuyorum = I am putting it under the chair
• çanta sandalyenin altında = the bag is under the chair

A useful contrast:

• -A / -e type endings often mean to / toward
• -DA / -de type endings often mean in / on / at

Since koymak involves putting something somewhere, altına is the natural choice.

It comes from the verb koymak = to put/place.

Breakdown:

• koy- = verb stem
• -uyor- = present continuous / progressive
• -um = I

So:

• koyuyorum = I am putting

This tense is often used for actions happening now or around now.

The progressive tense in Turkish is built with -iyor / -ıyor / -uyor / -üyor, depending on vowel harmony.

With koy-, the form becomes:

• koyuyor-

Then the 1st person singular ending is added:

• koyuyorum

So the ending tells you both:

• the action is ongoing
• the subject is I

That is another reason why ben is optional.

Because Turkish’s basic word order is usually Subject–Object–Verb.

This sentence follows that common pattern:

• Ben = subject
• çantayı = object
• sandalyenin altına = place/direction phrase
• koyuyorum = verb

So the overall structure is very natural for Turkish.

English usually puts the verb earlier:

• I am putting the bag under the chair.

Turkish typically saves the main verb for the end.

Yes, Turkish word order is flexible, although some orders sound more neutral than others.

The most neutral version here is:

• Ben çantayı sandalyenin altına koyuyorum.

But you could also say things like:

• Çantayı sandalyenin altına koyuyorum.
• Sandalyenin altına çantayı koyuyorum.
• Ben sandalyenin altına çantayı koyuyorum.

These versions can shift emphasis or topic, but the meaning stays basically the same because the case endings show the relationships clearly.

Yes, very roughly. Turkish often uses body-part- or region-like nouns for spatial relations.

Examples:

• üst = top / upper side
• alt = bottom / underside / under part
• iç = inside
• ön = front
• arka = back

So instead of using a separate word exactly like English under, Turkish often builds a phrase like:

• sandalyenin altı = the under-part of the chair
• masanın üstü = the top of the table
• evin önü = the front of the house

This is a very important Turkish pattern to get used to.

You could, but it would change the meaning.

• Çantayı sandalyenin altına koyuyorum. = I am putting the bag under the chair.
• Çanta sandalyenin altına koyuyorum. = this is not the normal form for a definite direct object and would sound wrong in standard Turkish for this meaning.

If you want an indefinite object, Turkish usually uses a slightly different structure, often with bir:

• Bir çanta sandalyenin altına koyuyorum. = I am putting a bag under the chair.

So in your sentence, çantayı is the correct form for the bag.

The dictionary form is koymak, meaning to put / to place.

In Turkish dictionaries, verbs are usually listed with -mak or -mek, which is the infinitive ending.

So:

• koymak = to put
• koyuyorum = I am putting

When you conjugate a verb, you remove -mak/-mek and build from the stem.

Here:

• koymak → koy- → koyuyorum