Nobody decides to take over. It happens because you are faster. You pick the breakfast, you set the order of the errands, you answer the phone before your parent reaches it, and you book the visitor because you already know the afternoon is free. Each choice is small and sensible. Added together, they move the decisions out of your parent's hands.
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The early signs
Listen for a shift in the sentences you hear. Whatever you think starts replacing an actual answer. Your parent stops correcting a choice you made. Visitors arrive without anyone asking whether the visit is wanted. Bills get paid in an order nobody discussed. None of this is a crisis. It is the early pattern, and it is far easier to reverse in the first month than in the fourth year.
Keep a short list of what stays theirs
Write down five or six decisions that belong to your parent by default: what to wear, what to eat for dinner, which chair the television faces, whether a neighbor stops by, what music is on, and when the bath happens if the bath is theirs to choose. Keep the list short enough to remember. Then ask before you decide on those items, even when the answer would have been the one you picked.
Compare the list with what you actually do on a normal day. The gaps show you where the drift is.
Separate preference from health
Some decisions involve health, and those are not yours to settle by guessing. Medicine stays with the family, the doctor and the pharmacist. If your parent seems to decide less or differently and you cannot tell whether the cause is health, mood, or simply a change in what they want, write down what you observed and the day you noticed it. Then put the question to the doctor in writing. Ask what the care team would like you to track and how they want changes reported. Keep the answer with the other records.
Ask one question before you decide
Pick one question you will ask every time, such as Do you want to do this, or should I choose? It sounds small, but its job is to keep the door open. If your parent answers you choose, accept it and let the offer stand. You are not trying to turn every choice into a contest. You are trying to make sure the choosing is still shared.