Was ist der Unterschied zwischen nachdenken und ueberlegen ?Du kannst auch einfach Beispielsätze geben.

I don't think there's any difference. It means exactly the same thing. However, in some cases it will sound more natural to use one of them and not either one. For example:

The imperative "think!" "Denk nach!" sounds more natural than "Überlege!" never heard the latter before.

He should consider it. Er sollte es sich überlegen is more natural than Er sollte darüber nachdenken.

überlegen is probably more about when you have to make a choice between a and b (and c) and nachdenken is when you are thinking very hard about something. but mostly not really any difference to be honest.