ABILITIES

                                            CAN / CAN'T

Auxiliary verb can (positive) - can't (negative) use

Use can, when you ask someone to do things. Use 'can' to talk about possibility. Always use can with another verb.
I can = I know to do something. / I know that something is possible for me.
Future: Use can if you are deciding now what to do in the future.

Can - can't statements and negatives

To form the negative add "not" after can to form one word. Example: cannot. Short form: can't. (can't = cannot)

Statements	Negatives
I can write a letter. You can write a letter. He / She can write  a letter. We can write a letter. You can write a letter. They can write a letter.	I can't write a letter. You can't write a letter. He / She can't write  a letter. We can't write a letter. You can't write a letter. They can't write a letter.

Now let's look at some examples :)