Subjunctive

The subjunctive is not a tense (e.g. present tense); rather, it is a mood. Tense refers to when an action takes place (past, present, future), while mood merely reflects how the speaker feels about the action. The subjunctive mood is rarely used in English, but it is widely used in Spanish.

The difference between subjunctive verbs and tenses

The subjunctive is used to express an imaginary situation, as opposed to a real situation. For example, we "recommended" that you read this web page. That was not a description of a real situation; it was an expression of our wish. Therefore, the mood is subjunctive. 

In English, the subjunctive mood is usually indicated by a subordinate clause, also called a dependent clause, beginning with the word "that". 

Verbs in the subjunctive mood tend to undergo slight conjugations in order to express this mood. 

Learning to use subjunctives in a sentence

Here is an example of the subjunctive mood being used correctly in a sentence. 

"It has been suggested that students refrain from drinking on the evenings before important exams."

In this sentence, "that students refrain" introduces a clause in the subjunctive mood. 

Now, here is an example of the subjunctive mood being used in an incorrect way. 

"The counselor said that the woman be more responsible than the man within the relationship, since she always kept her promises."

In this sentence, "be" is an incorrect use of the subjunctive, because the sentence is expressing a real and not an imagined situation. 

For your reference, here are a couple rules you can follow in order to make sure that you are using the subjunctive in a proper way. 

1. The subjunctive is almost always preceded by a phrase that indicates that the speaker is expressing not a factual state of affairs but rather his own wishes regarding what should be the case. This phrase is followed by the word "that", and then comes the dependent clause containing the subjunctive mood. 
2. Be careful to not get thrown off by the simple word "that": just because you see it does not mean the sentence is in the subjunctive (although almost all subjunctive sentences contain the word). "He said that" would not express the subjunctive, but "He suggested that" would. You have to pay attention to the sentence as a whole.   

Further reading

The existence of the subjunctive mood is premised on the idea that there is a meaningful distinction between what exists in reality and what exists in the imagination, or between an actual state of affairs and a possible state of affairs. As with several grammatical forms, this, too, could likely be traced back to the original uses of language when it first emerged within the species.

There can be a crucial, even life-changing, difference between reporting an actual situation in the world and expressing what situation one would like to see in the world. Clearly, it is important to not get these mixed up. That's why the subjunctive mood exists. 

In common English language use these days, though, it would seem that the subjunctive is a relatively rare grammatical construction. This is partly because one may not be certain when to make use of it, and partly because even the correct use of it can sound somewhat awkward to unaccustomed ears. More commonly, the word "should" may be used in order to convey the subjunctive mood.

For example, instead of saying you suggest "that someone go", you can say you suggest "that someone should go". They say the same thing, although one may sound more natural than the other for cultural reasons.