Using the subjunctive to express possibility / probability
De verás, el expresar de la probabilidad usando el subjunctivo (probability / possibility in the present, future, & past) me confunde mucho.
¿Hay alguien que pueda ayudarme a entender más claramente este concepto?
6 Answers
The simplest way to explain the subjunctive is to compare the informational content that is expressed by a verb in the indicative and subjunctive moods, respectively.
In the indicative mood, a verb can be considered declarative; that is, the information being conveyed is expressed as a declaration. Because the indicative is used to declare what the speaker perceives to be true or real, we might consider the informational content to be relatively high.
In the following two sentences, for example, the verb "estar" is conjugated in the indicative because in both cases in order to declare that "She is pregnant." Whether the person is actually pregnant or not is unimportant. What is important is to note that the statement is declared in accordance with the speakers view of reality.
?She is pregnant ? Está embarazada.
?I think that she is pregnant ? Creo que está embarazada.
Compare this with non-declarative sentences which often set up a sort of hypothetical condition:
?I doubt that she is pregnant ? Dudo que esté embarazada
Unfortunately, because of the way English is structured, a translation such as the one offered above does not convey well the non-declarative nature of the subjunctive in Spanish. It might help to break the sentence down into its two component clauses:
1). I doubt it.
2). She is pregnant
Here we see that from the speaker's perspective, the information that is represented by "it" (i.e. that she is pregnant) is looked upon as false. Because it is considered false, in Spanish we never present it with the indicative, i.e. as a declaration. That is, we cannot declare that we don't believe something to be true and then in the very same sentence declare that it is true. This might be a bit more understandable if we were to consider the effect of making the following two consecutive declarations:
(1). She is pregnant. (2). I doubt it.
Hopefully, you can see that it is somewhat paradoxical, to declare something to indeed be true only to refute the truth of your original statement in the very same breath. For this reason, in Spanish, the semantic value of the subjunctive can often be conveyed more accurately by the use of an appropriate modal auxiliary (could, should, would, can, etc) or by the use of an infinitive verb accompanied by a disjunctive pronoun. For example, the following Spanish sentence can be expressed a number of ways in English with varying degrees of faithfulness to the original non-declarative manner in which the it was expressed with the Spanish subjunctive:
No es posible que esté embarazada
1). It's not possible for her to be pregnant [infinitive]
2). It's not possible that she might/could be pregnant [modal aux.]
In both cases, no declaration is made in regards to the nature of her pregnancy. Compare, for example:
She is pregnant
versus
for her to be pregnant
or
She might/could be pregnant (i.e. She also might not be pregnant, but no definitive declaration is being made either way as to the actual disposition of her pregnancy)
It's not enough to assume that such non-declarative cases only occur when we are talking about what we believe or don't believe, however. Instead, it's important to point out that any time that we avoid making a declaration, we are in fact avoiding the addition of novel information. That is the information content that is conveyed by the subjunctive is very low. Take the following, for example:
Es posible que esté embarazada
It's possible for her to be pregnant ? This does not mean that she is indeed pregnant, only that the possibility for such a thing exists.
It's possible that she might be pregnant ? Again we are saying that the possibility exists, i.e. She might be pregnant, but by the same token, she might not be.
In both of the proposed translations above, we see that the non-declarative nature of the subjunctive does not allow us to view the idea of her being pregnant or not being pregnant as novel information. That is, she might be, or she might not be—a conclusion that does not need to be uttered to be readily apparent. Compare this with the declarative: "She is pregnant" or "She is not pregnant," two statements whose informational content is relatively high.
The idea of informational content and declarative vs. non-declarative can be helpful when we look at certain subordinate clauses in which the main clause expresses an emotion. Imagine for example that your wife has just told you that she is pregnant. Would it then be reasonable to "declare" to your wife that she is indeed pregnant (i.e. to present the information as novel information)? For example, would you make the following declarations:
I am so happy. [declaration]
You are pregnant. [declaration]
Of course not! But this is the exact form that our sentences often take in English:
I am so happy that you are pregnant
In Spanish, however, we avoid such strange ambiguities by using the designated subjunctive verb form:
Estoy tan feliz de que estés embarazada.
This allows us to present the subject matter as non-declarative, immediately indicating that the informational content is low or non-focal. Even though it would not be wrong to use the English translation presented above (in fact, it would not be a stretch to believe that this is probably the most common way to express this sentiment in English), for illustrative purposes, it might be helpful to observe an alternative translation which (while definitely less common) remains a bit more faithful to the non-declarative nature of the subjunctive:
Estoy tan feliz de que estés embarazada.
I am so happy for you to be pregnant.
I am so happy that you should be pregnant.
I have probably already run this into the ground, but at the risk of sounding repetitive, the best way to approach the subjunctive is by attempting to understand the idea behind why it is used, an idea that can best be described in terms of its non-declarative nature. When approached from this perspective, understanding how it is used, why it is used and when it is used becomes much simpler.
That being said, the absolute best way to know when to use the subjunctive is to focus on actual usage. That is, be sure to spend plenty of time listening to spoken Spanish, absorbing what you hear, and repeating it to yourself until you build up an echo in your head. If you do this long enough, analyzing "why" from the standpoint of grammar will become unnecessary. The ultimate answer to "why" is "because that is the way we say it. To become part of that "we" more quickly, it often helps to spend more time listening (and reading) to the language as it is spoken and less time focusing on abstract grammar concepts.
Of course you might take this last bit of advice with a grain of salt seeing as it is coming from a guy who himself enjoys sitting around twiddling with abstract grammar concepts. Whatever method of study you choose, best of luck in your endeavors 
Here's an example:
Indicative of estar, 2nd person (she) = está (she is)
Ella está embarazada. She is pregnant.
Subjunctive mood of the same tense - esté (she may be)
Es posible que esté embarazada. She may be pregnant.
Hi and welcome to the forum. Here is a link to the reference section on the subjunctive. I hope this helps.
On the subject of the future tense, it is interesting that some Spanish grammar books divide the non-imperative verb tenses according to mood (indicative, conditional, and subjunctive) and temporal or psychological perspective (present and past). In this scheme, the future tense is labeled the present form of the conditional mood and the conditional tense is labeled the past form. This is more in line with how the future tense is used in Spanish as opposed to how the future tense is used in English. It is usually best translated as "might" and not as "will".
Could any moderator add Iza's post to the reference section, please?
And this one as well?
¡Mil gracias a todos! Se lo agradezco su bonded a contestar mi pregunta.
However, I was looking for some more detail help explaining how the subjunctive works to express possibility or probability during a specific timeframe-- for ex. How "¿Será a las 3?" can mean 2 things 1) it will be at 3, 2) "it must be-/may be 3p"
