in which cases we use the subjuctive form of a verb? what is the subjunctive?

Answer 1

The subjunctive is not a 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 there are a few expressions which creep in, such as "the powers that be" where "be" is in the subjunctive mood, or "You dare!" which is not an order to the person you are speaking to to dare (quite the contrary!) but short for "if you were to dare..."
Here are some further example of the subjunctive:

The doctor recommends that he take the pills with food.
Subjunctive conjugation: he take

The law requires that you be 18 years old to vote.
Subjunctive conjugation: you be

If I were a rich man, I wouldn't have to work hard.
Subjunctive conjugation: I were

England expects that every man should do his duty. (Lord Nelson before the Battle of Trafalgar). It would have been possible to say "England expects that every man do his duty."

Answer 2

Ahem **subjunctive IS a tense** Though rarely used in English, all of the Romance languages have it (as well as Germanic ones, such as English)and it is especially emphasized in Spanish.

The subjunctive is difficult to understand because of its rarity in our language. When I teach it I usually call the subjunctive a "magical world of what-if." If you have struggles grasping it, don't worry, command of the subjunctive is something only true masters of language have. One Spanish professor told me that native speakers didn't grasp the subjunctive until age 10.

I explain it that way because the subjunctive tense is conjugated in a past, present, pluperfect and sometimes future tense. The subjunctive is really a group of tenses whose use are entirely dependant on the user.

examples

"I wish I were taller" --were is a subjunctive conjugation indicating a state based on what I wish.

"My mother prefers that I speak properly" prefers that is a compound verbal structure that is based on my mother. It doesn't mean I speak properly or that I will speak properly, it means that 'speaking properly' is in the perfect world my mother projects me. (by the way, it worked)

In American English 'was' has become the dominant subjunctive indicator; "I wish I was taller". It's not TECHNICALLY correct, but who the hell cares, we're Americans, right?

Answer 3

Subjuctive verbs show doubts, wishes and possibilities. For example, in the phrase, "if I were you", were is the subjuctive verb.

Answer 4

Hypotheticals in 'were' are subjunctive, e.g., "were I you". Compare this to indicative 'If I was you".