nachdenklich
thoughtful / pensive
Lost in thought and thinking deeply about something. A person in this state often appears quiet and serious.
Sein Vorschlag machte sie nachdenklich.
His suggestion made her thoughtful.

thoughtful / pensive
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thoughtful / pensive
Lost in thought and thinking deeply about something. A person in this state often appears quiet and serious.
Sein Vorschlag machte sie nachdenklich.
His suggestion made her thoughtful.

The word is derived from the verb 'nachdenken' (to think about, to reflect). The ending '-lich' turns it into an adjective that describes the state of a person who is currently thinking about something.
Nach dem Gespräch war er sehr nachdenklich.
After the conversation, he was very thoughtful.
A very typical construction is 'jemanden nachdenklich machen' (to make someone thoughtful) or 'jemanden nachdenklich stimmen' (to make someone pensive). This means that a piece of information or an event makes a person start thinking.
Der Film hat mich sehr nachdenklich gemacht.
The film made me very thoughtful.
'Nachdenklich' (thoughtful/pensive) often describes a person's facial expression or mood. Common combinations are 'ein nachdenklicher Blick' (a thoughtful look), 'eine nachdenkliche Miene' (a thoughtful expression), or 'nachdenklich werden' (to become thoughtful).
Mit nachdenklicher Miene blickte er aus dem Fenster.
With a thoughtful expression, he looked out the window.
serious
'ernst' and 'nachdenklich' can both describe someone who does not seem cheerful or relaxed. However, 'nachdenklich' mainly says that a person is thinking deeply about something and is therefore quiet. 'ernst' is more likely to describe a person's mood, facial expression, or manner: not joking, not funny, often serious. A thoughtful person can seem serious, but not every serious person is thinking deeply about something at that moment.

Nach dem Gespräch saß Lisa lange nachdenklich am Fenster.
After the conversation, Lisa sat pensively by the window for a long time.
Der Arzt machte ein ernstes Gesicht, als er die Ergebnisse erklärte.
The doctor had a serious expression as he explained the results.