I always love to help if I can. It gives me so much joy to help someone. I was not sure if I could explain it well but I took the challenge and tried to answer the question. Here is the answer that I wrote:
This is very easy if you think of the word "grandmother" or "grandma" as a title like "Mr.," "Mrs.," and "Ms." It is the title to show that they are a grandparent and that the gender is woman. Then we many times in English add the last name or surname to the title. For example: Grandma Smith if her name is Mary Smith or Grandma Jones if her name is maybe Susan Jones. We also sometimes use the first name instead of the last name depending on the Grandmother's wishes. Then the name for Mary Smith could be Grandma Mary or for Susan Jones it could be Grandma Susan. Usually the parents of the father or mother work this out if the name is the same. This is usually not a problem in our country since there are so many different names from so many countries.
Now let us take the name Grandpa or Grandfather for the title of a man who is a grandparent. Maybe his given name is John Doe or Michael White. This is how we might address them: Grandpa Doe or Grandfather Doe if his name is John Doe. Grandpa White or Grandfather White if his name is Michael White. We can also use their first given name to make Grandpa John or Grandfather John for the man named John Doe. In the same manner we could use Grandpa Michael or Grandfather Michael for a man named Michael White.
Many times people have favorite names as others have posted like nanna or papa. But the two paragraphs above is how we addressed grandparents for many years in the United States. A lot of the other forms have come along as our society has found it hard to accept that we get older or that we are getting old. There is a great emphasis on youth in the United States and very little emphasis on how important older people are to our society. A very sad thing to me.
I hope this makes it a little clearer. I am not sure if I explained it in terms that would translate well to your language and understanding.
The response from the person asking the question is that it really did help and that they understood now how we handle this. There were many responses to this question. I just love the internet that we can work together and help one another. Yes, there are uses for the internet that are not good but I think this is one use that really shines through. It is so exciting to get to know many kind people from all over the world.