keep, guard, save
Someone who is reserved is quiet, self-controlled, and keeps their thoughts mostly to themselves.
When you observe something, you watch it carefully to see what is happening.
When you reserve something, you keep it for yourself so that you are able to use it when you need to.
When you preserve something, such as a forest or food, you keep it from being destroyed or from harm coming to it by taking good care of it.
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