заду́маться

fairly common #1669B2

1.
to ponder, to consider, to wonder, to think (about something)
Я заду́мался над её слова́ми.
I pondered her words.
Often with над + instrumental or о + prepositional.
2.
to be lost in thought, to become pensive
Он заду́мался и не услы́шал вопро́са.
He was lost in thought and didn't hear the question.

Examples

Обкусивший карандаш студент задумался над задачей
The student, who had gnawed on his pencil, pondered the problem.
Извини́, я заду́мался.
Sorry, my mind was elsewhere.
Покрутившая кольцо на пальце девушка задумалась.
The girl who had twisted the ring on her finger became thoughtful.
Его резонное замечание заставило всех задуматься.
His reasonable remark made everyone think.
Вперявший взгляд в стену, он задумался о прошлом.
Having fixed his gaze on the wall, he began thinking about the past.
Тебе следует заду́маться о своем бу́дущем.
You should think about your future.
Капита́н умолк и глубоко заду́мался.
The captain fell silent and became lost in thought.
Философический роман заставил меня задуматься.
The philosophical novel made me think.

Imperative
ты	заду́майся
вы	заду́майтесь

Active present	-
Active past	заду́мавшийся	pensive, thoughtful, lost in thought
Passive present	-
Passive past	-
Gerund present	-
Gerund past	заду́мавшись	while doing (past)

ду́матьto think; to think; to intend
verbimperfective
уду́матьto think up
verbperfective
взду́матьto take it into one's head; to not dare (do sth)
verbperfective
вы́думатьto think up; to make up
verbperfective
доду́матьto think through (to the end); to think up what is missing
verbperfective