Latin/Lesson 1-Subjunctive

Subjunctives.
The Subjunctive is one of the three different moods a Latin verb can take. The two other moods are the Indicative and the Imperative. The subjunctive is perhaps the most common and also most difficult to grasp, and there are a great number of different subjunctive uses.
The subjunctive mainly expresses doubt or potential and what could have been. Whereas the indicative declares "this happened" or "that happened," the imperative is called 'jussive,' which is from 'iubere' - to command, bid.
""If" this "were" to happen," or "May this happen!" or "I ask you to make this happen" are all possible uses of the subjunctive.
There are four subjunctives: present, imperfect, perfect, and pluperfect. There are no subjunctives in the future tense, which already incorporates an element of doubt.
The Present Subjunctive.
The present subjunctive is similar to the present indicative, except marked by a change of the theme vowel.
"present stem" + "theme vowel change" + "ending"
Form.
In the present subjunctive, the theme vowel for every conjugation changes; in effect, the first conjugation masquerades as the second conjugation and all the other conjugations take on the appearance of the first.
Some ways to remember this are in the following collapsed table.
Example Conjugation.
"porto, portare, portavi, portatus" (1st conjugation - to carry)
Present Indicative.
This is the present active indicative form of "portāre", which has already been covered.
Remember the join vowels.
Present Subjunctive.
The present active subjunctive of "portare" would be conjugated as follows: 
Notice:
Present Subjunctive of Esse.
The present active subjunctive of "sum, esse", the verb "to be", is conjugated as follows: 
Unlike the conjugation of the present active indicative form, the present subjunctive is regular. The same personal endings are affixed to "si-".
Present Subjunctive of Posse.
Translates as "May I/you/he/we/you/they be able"
Imperfect Subjunctive.
The imperfect subjunctive is formed by adding the personal endings "-m, -s, -t, -mus, -tis, -nt" or the passive endings "-r, -ris, -tur, -mur, -mini, -ntur" to the present infinitive active (often the second principal part). In other words, for
"vocō, vocāre, vocāvī, vocātum"
The imperfect subjunctives are formed thus:
For deponent verbs, whose second principal part is the passive infinitive (e.g., cōnārī, verērī, patī, expedīrī) a pseudo present infinitive is used (e.g, cōnāre, verēre, patere, expedīre; although these forms do not exist as stand-alone infinitives, they actually ARE the singular imperatives for these deponent verbs)
The imperfect subjunctive of the verb to be (sum, esse) is conjugated regularly, as are ALL irregular verbs, e.g.: possem, vellem, nollem, ferrem, irem)
Uses of the Subjunctive.
Verbs in the subjunctive mood may assume special meaning in specific constructions.
Volitive or Optative Clauses.
Subjunctives in independent clauses are often translated as volitive/optative (that is, as a wish). Volitives/optatives show an intention for an action to occur; e.g. "amet" may be translated in volitive/optative context as "may he love"
Hortatory.
A suggestion or command in first person (most often plural); e.g. "cedamus" as an hortatory subjunctive is "let us depart"
Jussive.
A suggestion or command in third person; e.g. "cedat" as a jussive subjunctive is "let her depart"; "deprehendatur"= "Let him be seized"
Potential.
The potential or possibility of something happening, in any person: (Fortasse) te amem. "Perhaps I may love you."
All of these Subjunctive types can be used in an independent (main) clause. 
Note that all can be translated with "let" or "may"; the differences lie in how English will represent the subjunctive verb:
Purpose Clauses.
A purpose clause is a dependent clause used, as the name shows, to show purpose. Often initiated by an indicative verb, the clause contains a subjunctive verb in either the present or imperfect tense. Present and imperfect verbs in purpose clauses should be translated with the auxiliary verbs "may" and "might," respectively. For example, "Marcus urbem condidit ut regeret" should be translated as "Marcus built the city so that he would rule." These appear frequently in Latin.
Relative Clauses.
A relative clause is an independent clause introduced by a relative pronoun. When the verb of a relative clause is in the subjunctive mood the clause may express result, purpose, or characteristic.