to think
The word 'penser' in French is equivalent to 'to think' in English. It is used to express the action of producing thoughts or ideas. 'Penser' can be used in various contexts, just like the English version, either about concrete thoughts or abstract concepts. For instance, 'Je pense que c'est une bonne idée' which translates to 'I think it's a good idea'.
I think you are smart.
This sentence is an example of using 'penser' in a subjective sentence to express opinion. The verb 'pense' is in the first person singular form 'Je pense', which means 'I think'.
He is thinking about going to the cinema tonight.
This sentence uses 'penser' in context of planning or considering an action in the future. 'Il pense' is used in third person singular which translates to 'He thinks' or 'He is thinking'.
What do you think about this book?
In this phrase 'penser' is used in the context of asking for someone's opinion. The phrase 'Que penses-tu' is a way of asking 'What do you think' in French.