应该看英文的语法书或网站或教程，比如牛津语法词典的说法是：
subjunctive (n. & adj.) Traditionally: (a *verb form or *mood) expres-
sing hypothesis or non-factuality. Contrasted particularly with the *indic-
ative. Also called conjunctive, especially in grammars describing ancient
languages.
The present subjunctive form of a verb is *finite, and identical with the
*base (1) form of the verb. Formally, it is exactly the same as the present
tense indicative form, except for the *third person singular, which lacks -s,
and for the verb be, whose present subjunctive form is be. Functionally, it
can refer both to the present and to the past. The present subjunctive can
be used in three different ways, described below.
First, the mandative subjunctive (or subjunctive mandative) is used in
*subordinate clauses following an expression of command, necessity,
suggestion, or possibility, e.g.
I recommended that he write and apologize
She requested that she not be disturbed
Notice that the negative element not is positioned before the verb in the
second example.
It is possible to have should in the subordinate clause:
I insisted that he should attend the meeting
This is called the should-mandative in CaGEL, and *putative should in
CGEL.
Secondly, the present subjunctive can be used, rather formally, in
subordinate clauses of *condition and *concession, but not with past
reference, e.g.
If that be the case, our position is indefensible
Thirdly, the formulaic or optative subjunctive is used in independent
clauses, mainly in set expressions, e.g.
God save the Queen
Some such clauses have an unusual *word order, e.g.
Perish the thought!
Come hell or high water
The so-called past subjunctive (also called the were-subjunctive or
*irrealis were) is used in clauses of hypothetical condition. It differs from
the past indicative form of be only in the first and third person singular,
where were is used, though was is increasingly found here too. The
reference is to present (or future) time, e.g.
If I were you, I’d own up (compare: If I was you . . . )
If only my grandfather were alive today (compare: If only my grandfather was . … )
If she were to come tomorrow . . . (compare: If she was to . . . )
The uses of ordinary indicative forms to express non-factuality, such as
the use of a *past tense to refer to a present or future situation (e.g. If
you came tomorrow . . . ; see past (2)), have been described as subjunctive
uses—perhaps because in translation such a usage might need a
subjunctive form in another language. Modern grammar considers this to
be quite unjustified, and restricts the use of the term subjunctive as
described above.
However, since Modern English (unlike, say, French or Spanish) has
few distinct verb forms that differentiate subjunctive verb forms from
indicative verb forms, the status of the subjunctive as a verbal *inflection
has been challenged. Indeed, many modern frameworks prefer to speak of
subjunctive constructions or subjunctive clauses. These labels then
apply to the entire clause in which the ‘subjunctive verb’ appears.
The disappearance of the subjunctive has long been forecast:
1860 G. P. MARSH The subjunctive is evidently passing out of use, and there is
good reason to suppose that it will soon become obsolete altogether.
But it survives, and indeed has been seen to be on the increase, especially
the mandative subjunctive in British English in recent years, possibly
under American influence.
● ● covert subjunctive: a term used in CaGel to describe a construction in
which an indicative verb form is used where a ‘subjunctive’ verb form
might be expected, given the meaning of the *governing verb, e.g. They
insist that he eats his dinner (cf. They insist that he eat his dinner).
See also putative should.