Pensar [en]
 

Hi,

Pensar en means to think about, they say. When am I supposed/allowed to drop en?

¿Nos tapamos la nariz, damos tres
vueltas con los ojos cerrados y pensamos cosas
buenas?

When there is a sense of reflection or meditation involved.
-Pienso en ti todos los días.
-He estado pensando en lo que te dije ayer.
-¿En qué piensas? (A penny for your thoughts)
-Siempre pensamos en cómo explicar mejor las cosas
-Mis padres siempre pensaban en el futuro.

Thanks! My question is more about the cases without en, as in the example.

Let me think about this. I'll come back to you. :)
In the meantime, maybe some other users will have an answer. ;)

The only thing I can think of is that to think about in Spanish is pensar en, but you can optionally drop the en in front of some nouns like cosas if the noun refers to something nonspecific or abstract.
It can work this way in English. Example: think about good things/think good things....think puppy dogs, think roses, think seashore.

This is an interesting question. Now it has got me thinking what the difference is between these two:

Pienso muchas cosas.
Pienso en muchas cosas.

@Tyrn One case where you don't need to use "en" after pensar is if you use "que" after pensar. "Pienso que..."

I'll try to explain from what the "Diccionario Panhispánico de Dudas" says:

a) When "pensar" means to believe, to intend, to decide, it is a transitive verb. It means it has a Direct Object:
- Pienso (creo) que debes hacer muchas preguntas.
I think you must ask many questions.
- Se piensa (cree) que es fácil explicar la gramática del español.
It is believed that explaining Spanish grammar is easy.
- Pensábamos (teníamos la intención de) ir al cine, pero llovió y ya no salimos.
We intended to go to the movies, but it rained and we didn't go out.
- El gobierno ha pensado (decidido) comprar más vacunas.
The government has decided to buy more vaccines.
- Hemos pensado (concluido) que no necesitamos comprar una casa.
We have come to the conclusion we don't need to buy a house.


b) The verb "pensar" needs the complement "en" when it means to remember, to reflect on something, to have in mind something/someone, and then it becomes an intransitive verb:
- Todavía pienso en (recuerdo) mis últimas vacaciones.
I still remember my last vacation.
- No he pensado (reflexionado) en lo que voy a hacer cuando me retire.
I haven't thought about what I'll do when I retire.
- Deberías pensar más en los demás.
You should think more about other people.
- Pienso mucho en los años que pasé trabajando sin descanso.
I think a lot about the years I spent working non-stop.
- Roberto nunca piensa en su familia.
Roberto never thinks about his family.
- Mis hijos sólo piensan en jugar videojuegos.
My children can't think of anything but playing videogames.

* But, sometimes, to be more "poetic", people take "pensar" meaning to remember something/someone or to reflect on something as a transitive verb, and get rid of "en":
- No te dejo de pensar. / Te pienso mucho.
I can't stop thinking about you. / I'm constantly thinking about you.
- Pienso lo mucho que te debo y estoy muy agradecido.
I think about how much I owe you and I'm so thankful.

*Normally, I find this usage wrong, but the Academia is not bothered about it, because it's very commonly found.
This is the case of "piensa cosas bonitas". At first, it didn't sound right to me (I would have said "piensa en cosas bonitas"), but couldn't explain why, so maybe that's why I didn't actually reply to your question.
Sorry for that. I hope this will be clearer. :)




@Tomísimo: They may mean the same (according to the last note), but if you ask me, "pienso muchas cosas" would mean "tengo opiniones sobre muchos asuntos", and "pienso en muchas cosas" means "tengo muchos asuntos sobre los cuales estoy reflexionando". :)