Subjunctive vs. Infinitive when subject is switched

Question

I just have a question about a detail on indicative vs. subjunctive:

I know that one grammar rule is that the subjunctive tense is used in a sentence where the subject of the independent clause is different from the subject of the dependent clause. However, indicative is used with expressions that do not express any doubt (e.g. those that begin with "es cierto", "pienso","sé", etc.).

However, what if we had a sentence where both are used, like: "Pienso que tú ___ (pensar) cuidadosamente antes de contestar mi pregunta." Would we use the indicative "piensas" or the subjunctive "pienses" in this situation?

Also, this is a very silly example that doesn't make a whole lot of sense, so if anyone has other examples that could help clarify my question they would be appreciated.

Answer 1

The rule is not exactly as you put it, but I get what you mean and it's a good rule of thumb. The thing is, as you already know, it depends on the main verb. It makes no difference what the verb is in the subordinate sentence.

 MAIN        SUBORDINATE 
 VERB           VERB
  ↓              ↓
Pienso [ que trabajas mucho ].  "I think you work hard." 
Quiero [ que trabajes mejor ].  "I want you to work better."

Your fill-in-the-blank example is not something you'd find in real conversation. With pensar as main verb (and so long as it is in the affirmative), the verb in the subordinate will be in the indicative. So you would get this:

Pienso que tú piensas cuidadosamente antes de contestar mi pregunta.

That's grammatical but makes no sense. At first it reads like a general statement ("I think you always/generally/habitually think carefully...") but when you add «antes de contestar mi pregunta» it becomes definite and cannot be read as general anymore.

Another possibility is using the future indicative:

Pienso que pensarás cuidadosamente antes de contestar mi pregunta.

That is, "I think you will think carefully before answering my question".

Moreover, in all the examples above, pensar sounds a bit forced; most people will use creer instead («Creo que...»). There's a good answer elsewhere to the question about the difference between creer and pensar.

Answer 2

In your example sentence, because the subject believes the listener thinks something, the subordinate clause is treated as factual and therefore must be in the indicative mood.

Interestingly, if the main clause were in the negative ("I don't think you think... "), the subjunctive mood would be mandatory for the subordinate clause, because in this case it would refer to something not believed to be true.

A more general and helpful rule is that the subjunctive is reserved for non-factual statements: hypotheses, wishes, commands, conditions, and the like.