Can I say kaddish for a grandparent who passed away 20 years ago? Can I use his/her English names? Can I say kaddish even not their Yortzheit?

Shalom, Thank you for your question. The Kaddish prayer is usually recited by the mourners – and not the grandchildren. However there are special circumstances where a grandchild would say kaddish for a grandparent. In any event – kaddish for the deceased (after the first year) is only said on the Yortzeit. So, in answer to your question – kaddish for someone who passed away 20 years ago should be said only on their Yortzeit. Usually it will be said by the remaining children. However, if they are not saying kaddish (for whatever reason), then a grandchild would be permitted to say kaddish. But, in general a child whose parents are still alive should not say kaddish. So, if your parents are still alive you would need to get permission from your parents to say kaddish for your grandparent. As to using their name – it is not clear exactly what the question is. If you are asking may you name your children after your departed grandparents, the answer is of course yes. Generally we name after people using the Hebrew name. But it is permitted to also use their English name. So, a person may name their child in English Henry after their grandfather Henry – but it would be a greater merit to also name the child in Hebrew Chaim after the Hebrew name Haim. I hope this is of some help - Blessings.