The subjunctive mood appears in only a few contexts:
In contrary-to-fact clauses beginning with if
When a subordinate clause beginning with if expresses a condition contrary to fact, use the subjunctive mood.
The verbs in these sentences express conditions that do not exist: The writer is not a member of Congress, and Jake is not trustworthy.
Do not use the subjunctive mood in if clauses expressing conditions that exist or may exist.
In contrary-to-fact clauses expressing a wish
In formal English, the subjunctive is used in clauses expressing a wish or desire; in informal speech, however, the indicative is more commonly used.
In that clauses following verbs such as ask, insist, recommend, request, and suggest
Because requests have not yet become reality, they are expressed in the subjunctive mood.
In certain familiar expressions
The subjunctive mood, once more widely used, remains in certain set expressions: be that as it may, as it were, far be it from me, and so on.
Exercise: Verb tense and mood 1
Exercise: Verb tense and mood 2
Exercise: Verb tense and mood 3