Correct usage of the Subjunctive?
I was reading the reference page on the subjunctive and the first point it makes is that a sentence using the subjunctive must have two clauses with different subjects, but I have seen sentences using the subjunctive with out changing the subject between the two clauses, and so I am wondering if these following sentences are correct uses of the subjunctive. - "Quizás vaya a casa." Perhaps I will go home. - "Habla como si fuera inteligente." She talks as if she were intelligent. - "Si yo fuera un rico, tocaría el violín." If I were a rich man, I would play the fiddle. - "Es probable que salga tarde." It is likely that it will leave late.
3 Answers
It might help to think of the difference between the subjunctive and the indicative in terms of the difference that exists between speaking about a hypothetical situation and making a declarative statement.
"Quizás vaya a casa." Perhaps I will go home.
The word "quizás" (and other similar expressions) is a word that can be accompanied by either the subjunctive or indicative depending on your level of certainty. For example, with the subjunctive, you are expressing a hypothetical situation. You are not declaring that you "are" or "will" go home with certainty, but only that you might go home (i.e. there is a possibility yet still quite a bit of uncertainty).
"Habla como si fuera inteligente." She talks as if she were intelligent.
Here this is the same as before. The speaker is not declaring that the person (she) is speaking with intelligence but instead is using the subjunctive to describe a hypothetical/contrary to fact condition (i.e. "as if she were"). It might help you to know that in English, the subjunctive is always follows an if statement that is preceded by the word "as" (i.e. as if she were intelligent, as if we were friends, as if I knew what I was talking about).
"Si yo fuera un rico, tocaría el violín." If I were a rich man, I would play the fiddle. -
Again, this is an expression of a hypothetical/contrary to fact condition. The speaker is not making a declaration in regards to being rich (i.e. he is not saying I "am" a rich man [indicative] but instead is postulating what might be "were" he rich [subjunctive]).
"Es probable que salga tarde." It is likely that it will leave late.
The interpretation which you have posited sounds a bit odd. I would think that it might be better to read this as "that I/he/she will leave late" rather than "it will..." In any event, the "es probable" at the beginning of the sentence sets up the hypothetical/non-declarative intent (i.e. the speaker is not declaring that the person "is" leaving tomorrow but that the person "might/could" leave tomorrow.
I am not sure if this will make sense to you or not, but I can give you a bit of advice in regards to the use of the subjunctive. It might help you to look at a few sources in depth which relate to the subjunctive in terms of English grammar. The fact is that much of what is said about the subjunctive in English can be extended to help you better understand the use of the subjunctive in Spanish. The main difference (in my opinion) between the use of this mood between the two languages is that in English, because of the more limited morphology of the verbs (i.e. the designated verb tenses for the subjunctive present tense are only readily discernible from the indicative in the third person singular verb form). What English does to compensate for this limitation is employ either a modal auxiliary (words like might, could, should, would, can, etc) or to use an infinitive verb form.
For example:
| Spanish | Literal | Common English Expression | |
| Quiero que lave el coche | I want that she wash the car | I want her to wash the car [infinitive] | |
| No creo que le haga daño | I don't that believe that he do her harm | I don't believe that he would hurt her [modal auxiliary - would] |
must have two clauses with different subjects
Bottom line: the author shuold not have used the words "must have".
Here is how other authors express it: subjunctive mood
It is usually used in subordinate clauses that begin with que or si, although in some sentences (such as simple sentences expressing doubt), a subjunctive verb can be the main verb in the sentence.
Another article on subjunctive mood
The subjunctive mood is found primarily in dependent clauses, but of course the other moods can occur there as well, depending on the type of clause, the action/state involved, and its relationship to other elements in the sentences such as the governing verb.
Use of the subjunctive in independent clauses. As already mentioned, the subjunctive is seldom used in main clauses, but there are a few exceptions: (you can read the article to see the exceptions).
Here is another author that also likes the word must: See the 3 conditions that he says must happen to use the subjunctive. According to these conditions que is the only word that can start the dependent clause. That would rule out clauses starting with si, cuando, en cuanto, etc.
So take some of these comments by authors with a grain of salt. They tend to make generalized statements, leaving you puzzled when you encounter the exceptions.
After reminding students that the subjunctive is a mood rather than a tense which reflects the feeling of uncertainty that someone or something has done/is doing or will do a particular action, studyspanish.com had this helpful paragraph regarding when to use the subjunctive:
If you encounter a sentence with a main clause followed by a second clause, and the main clause does not introduce a quality of certainty or objectivity, the sentence will usually use the subjunctive mood in the second clause, since the sentence will not be reporting something certain.
As you rightly pointed out, although most commonly used in sentences with two clauses the subjunctive can also be used in independent clauses - perhaps the reference article may need a slight rewording 
This articles has a good explanation including some exceptions to the usual use of the subjunctive in sentences with 2 clauses and two different subjects.
The Use of the Subjunctive in Spanish
Use of the subjunctive in independent clauses. As already mentioned, the subjunctive is seldom used in main clauses, but there are a few exceptions:
The past subjunctive may be used with the verbs poder, querer, and deber to express courtesy or an attitude of deference. (Yo) quisiera pedirte un favor. I'd like to ask you a favor. Debieras practicar un poco más. You should practice a little more. ¿Pudieran Uds. darme otros ejemplos? Could you give me some other examples?
Words meaning “perhaps” (tal vez, quizá, and quizás) may be followed by the subjunctive to suggest that the action or state is improbable, or doubtful; they may also be followed by the indicative to stress a greater degree of likelihood or probability.
[Note: The expression for “perhaps” has no effect on the verb if it comes after the verb.]
Quizás participamos en la fiesta. Perhaps we'll take part in the festival. [Indic.: probable] Quizás participemos en la fiesta. Perhaps we'll take part in the festival. [Subj.: doubtful]
[1]: http://users.ipfw.edu/JEHLE/courses/persuad1.htm
