When you refer to ability, it can be in the present or in the past.
If you talk about abilities in the present. you must use CAN/ CAN'T=CANNOT. In the past you need to use COULD/ COULDN'T.
Ex: When i was 10 years old, i could ride a bike.
Now, i am 18 years old, i can use a computer. I can drive a car.
Usually, they use the past ability with the past of to be ( was, were).
To build the short or full answers, you need to know the questions, the formula is:
Can / could + subject + main verb + object?
The formual of shor answers are like that:
Yes, + subject + can.
No, + subject + can't.
For the full answer, the formula will be:
Yes, + subject + can + main verb + obj.
No, + subject + can't + main verb + obj.
Ex: Can you play soccer? Yes, i can or Yes, i can play soccer.
No, i can't. or No, i can't play soccer.
