I am going to learn c# and unity3D but i am not sure if i can publish a commercial game using only unity free.
I am asking about unity 4.
Yes, you can sell games with the free version:
Making and selling games with Unity
Can we sell games and make money with the free version of Unity?
Yes you can create and sell a game with the free version of Unity, without paying royalties or any revenue share. However, the free version of Unity may not be licensed by a commercial entity with annual gross revenues (based on fiscal year) in excess of US$100,000, or by an educational, non-profit or government entity with an annual budget of over US$100,000.
Please also be aware that the feature set of the free version is not intended for the production of professional games and interactive content. Lastly, games made with our Pro trial licenses may not be distributed nor used for commercial purposes.
Read more in the Unity FAQ section.
Good luck!