The subjunctive mood is a verb form that is used to express doubt or uncertainty in a statement. When someone says something that is doubtful or that is clearly known to be false, the subjunctive mood changes the form of the verb to express that feeling. Another time when the subjunctive mood is used is to convey that a speaker is stating something that is a wish, hope, or of uncertain outcome. This verb in such circumstances is different than the verb that would be used in the indicative mood, even if the rest of the sentence is a declarative statement.
Conditional sentences or statements are the most common and recognizable use of the subjunctive mood. A phrase that begins with if and then states a condition that is either uncertain or known to be false should contain a verb in the subjunctive mood. For example, someone might say, "If I were rich, I would" which indicates a condition that is known to be false and an idea that is dependent on that condition. The verb "were" is the present subjunctive form of the verb "to be."
The expression of a wish or hope, or the statement of an outcome that is uncertain also calls for the use of the subjunctive mood. The phrase "I wish" is typically followed by another phrase that is not yet true or that may not happen. This uncertainty is expressed by using the subjunctive form of the verb.
Changing the form of the verb in this way might seem grammatically incorrect because it uses what is ordinarily the past tense form of a verb in a present tense sentence. For most native speakers, the change in form simply sounds right. It isn't until someone stops to think about it or explain it to someone else that many native speakers will ever even notice the difference. On the other hand, people who are new to the language can sometimes find the subjunctive mood rather confusing, especially when hearing it used for the first time.
There are however, instances when the verb does not seem changed because of how a specific verb is conjugated for a specific subject. An example of this would be saying "you were" compared to saying "she were" when expressing a wish or hope. Adding "I wish" to the beginning of either phrase makes it correct in the subjunctive, but the phrase "she were" sounds much more awkward by itself because it is so different from how that particular subject and that particular form of the verb are normally used together.