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How to ask about a parent's previous marriage
Make a habit of chatting with your parent regularly.
Tell your parent about interesting things that happen to you, and ask them how their day is going. You don't have to talk about big, important topics.
Little conversations, like telling a funny story or discussing what to have for dinner, can also help you keep the channels of communication open. If you don't currently talk to your parent much, establishing this habit will probably take time.