hello
i want to ask about the suffix (i)when it means the direct object like (the car ), is it the same to say :
1.sell the car, 2.sell a car, with the same turkish sentence
arabayaı sat.
thanks so much.

i think this is right..

bir araba sat= sell a car
arabayı sat= sell the car

The direct object case (the accusative case) is used with nouns that are objects of transitive verbs. A transitive verb is a verb that can be applied to an object. For instance to sleep is intransitive but to love is transitive since the former doesn't take an object while the latter does.

In other words, we add -i(or -ı,-u,-ü,-yı,-yi,-yu,-yü,) if an action applies to a certain object.

loving İstanbul. > İstanbul'u sevmek.
I love İstanbul. > İstanbul'u seviyorum.

selling a (particular) car. > arabayı satmak*
I want to sell the car. > Arabayı saymak istiyorum.
I'm going to sell this car. > Bu arabayı satacağım.

*Notice that "araba satmak" is also possible but will have a different meaning.

Araba satmak benim en iyi yaptığım iş.
Selling cars is the best thing that I can do.

You need to know which verbs are transitive so you can use the accusative at the right place. For instance özlemek (to miss) and sevmek (to like/love) are transitive. This means if they take an object the object is always in accusative case.

The accusative also applies to verbal nouns. Verbal nouns are usually constructed with the -ma suffix.

okumak : infinitive (to read)
oku : verb stem (read)
okuma > verbal noun (reading)
Kitap okumayı sever misin?

Denizde yüzmeyi özledim.
Denizi özledim.