Keep in mind that pensar is conjugated irregularly. When the stem pens- is stressed, it becomes piens-. Thus, the present indicative forms are pienso (I think), piensas (you think), piensa (he/she/you think), pensamos (we think), pensáis (you think), piensan (they/you think).
Here are the main uses of pensar:
- Used by itself, pensar typically means "to think."
- Examples: Pienso, luego existo. (I think, therefore I am.) No pienso mal de ellos. (I don't think bad of them.)
- Pensar que is a very common way of indicating opinions or beliefs. In the positive form, it is followed by a verb in the indicative mood. Note that while que in this usage ususally can be translated to English as "that," it often can be left untranslated, as in following examples.
- Examples: Pienso que vivo como un cerdo. (I think I live like a pig.) Mi madre piensa que el doctor es culpable. (My mother believes the doctor is at fault.)
- When used negatively, no pensar que is followed by a verb in the subjunctive mood.
- Examples: No pienso que seamos diferentes. (I don't believe we are different.) No pensábamos que fueran a darnos problemas. (We didn't think they were going to give us any problems.)
- When followed by en, pensar typically means "to think about" in the sense of having one's thoughts focus on something.
- Examples: Estoy pensando en ti. (I'm thinking about you.) Pablo no piensa en los riesgos. (Paul doesn't think about the risks.) Las chicas sólo piensan en divertirse. (The girls think only about having fun.) Nadie piensa en cambiar las baterías. (Nobody thinks about changing the batteries.)
- Pensar de also is another way to say "to have an opinion about."
- Examples: Esto es lo que pienso de tu regalo. (This is what I think about your gift.) Tenemos que cambiar de lo que pensamos de nosotros mismos. (We need to change what we think about ourselves.)
- When followed by an infinitive, pensar is used to indicate plans or intentions.
- Examples: Pensamos salir mañana. (We're intending to leave tomorrow.) Yo pienso estudiar medicina de veterinaria en la universidad. (I'm planning on studying veterinary medicine at the university.)
- Pensar sobre can mean basically the same thing as pensar en but is much less common and is probably overused by English speakers speaking Spanish as a second language, or when translating from English to Spanish.
- Examples: Pienso sobre eso día y noche. (I think about it day and night.) Primero hacen y luego piensan sobre ello. (First they acted, and then they thought about it.)
- Pensar sobre can also mean to have an opinion about, especially when used in a question. Pensar de is more common.
- Examples: ¿Que piensas sobre la nueva web? (What do you think about the new web site?) ¿Qué piensan sobre los ataques suicidas como instrumento táctico para ser utilizado en una guerra? (What do they think about suicide attacks as a tactical instrument to be used in a war?)