Mine is a possessive pronoun (this sheets are mine) while,on the contrary, My is defined as a possessive determiner (My parents are coming to visit "my" flat this week-end, to bring me some clothes which are "mine")
To sum up :

My : Used before a noun

Mine : in place a noun

Bye ! :)

It depends on the sentence structure. Similar to spanish (I don't know portuguese)

This is MY house. (Esta es mi casa)

This house is MINE. (Esta casa es mio)

Is this my shirt? (Esta es mi camisa?)

Is this shirt mine? (Esta camisa es mio?)

Hello! First of all, look at these two examples:

This is my car.
This car is mine.

We use "my" (your, her, his, our, their) before the noun. We use "mine" (yours, hers, his, ours, theirs) after the noun. It all comes down to what you want to emphasize the most. Suppose that I've found a key, but I don't know who owns this key. So, I'm going to ask: "Did anybody lose a key here?" And then you say: "This key is MINE". Now, let's think about another situation: you're going to show me your house, and then you start introducing me your relatives. You may say: this is my FATHER, this is MOTHER, etc. Did you understand it?

They both mean the same thing but are applied in different situations:

Usually you use "mine" when answering a question. EG:

"Who's pen is this?"

"It's mine"

If you are just talking about yourself, you usually use "my" EG.

"This is my dog"

"That's my house"

You would never use "mine" in this situation, such as "that's mine pen".

However, you can also use "my" to answer a questions EG

"Who's pen is this?"

"That's my pen"

Hope this helps!