A. When did you start playing video games?

I don't know well​. --> I forgot. => Another way: Your sentence is okay and this is another way of saying it.

​

Please try to elaborate your answer.

​

Well, I'm not sure. I have to make a plan. -- > Well, I don't know. I have to make plans. => Your sentence has a correct grammatical structure, but this has the correct usage/ expression.

​

 Deokbo, what are you thinking? -- > Dugbo, what are you thinking? => Corrected spelling.

​

I have an idea. I'm going to my grandmother and grandfather's house. -- >  I have a good idea. I'm going to visit my grandparents.  => Your sentence has a correct grammatical structure, but this has the correct usage/ expression.​