Present Tense: think / thinks

• What does he think about his class?
• He thinks it's difficult.

Present Continuous Tense: (be) thinking

• What's he thinking about?
• He's thinking about the answer to a question.  

Yves is thinking about the answer to a question.

Past Tense: thought

• He thought he knew the answer.

Infinitive: It's necessary to think about your future goals in education.

Gerund: Thinking about and planning for the future is a smart thing to do.