Thanks masaegu san. :)

I get it now. Thanks alot.

I have another question which I hope you don't mind me asking. For example, I am about to tell you that I bought a dog and this information is something new to you. Am I right to say:

僕が犬を買いました。

No. It's は.

You are not new in any way because people know you already. It's the fact that you bought a dog that is new.

To describe a fact like that that is news to the listener, you make a change in the verb, not the particle after the subject.

犬を買ったんです（よ）。
犬を買ったんだ（よ）。

If you want to use a subject pronoun, use ボクは.
________

If someone asked you, "Who bought a dog?", then you could say:

僕が犬を買いました。

はい、分かりました。I am going to jot this important piece of information into my notebook. Thanks for the help, masaegu san.