Pensar (to think, to plan) requires either a preposition, a conjunction or an infinitive.
Pensar en: to think about, to have something on your mind
| ¿En qué piensas? | What are you thinking about? |
| Pienso en la película que vi anoche. | I'm thinking about the movie I saw last night. |
Pensar de: to have an opinion about (regarding)
| ¿Qué piensas de la película? | What do you think about the movie? (What is your opinion of the movie?) |
| Pues, depende de lo que Antonio piensa de la película.. | Well, that depends on what Antonio thought about the movie. |
Pensar que: to think that..... (expressing your opinion)
| Pienso que Antonio Banderas es requete sexy. | I think that Antonio Banderas is super sexy. |
| Pienso que es mejor ver la película otra vez. | I think that it's best to see the movie again! |
Pensar + infinitive: to think about doing something, to plan
| ¿Qué piensas hacer este fin de semana? | What are you planning to do this weekend? (thinking about doing?) |
| Pienso visitar a mi amiga en Walnut Creek. | I'm planning to visit my friend in Walnut Creek. (thinking about visiting my friend in Walnut Creek) |