What is the opposite of 'subjunctive'?

The short answer to your question is "mood", like the indicative.

John Lawler comments that "English has no subjunctive mood". I do not entirely agree with him, although I agree with him that in Latin, at least, subjunctive is a mood.

It probably depends on your country and your age! I am aged 64 living in London, UK. Most of the people I talk to in an average week use the subjunctive and use it correctly. But it is probably correct that a majority of native English speakers in the UK no longer use it or use it incorrectly.

50 years ago in the UK, the top schools taught Latin. It was considered to be the best way to learn to write and speak English properly and to learn to think clearly. Today few schools in the UK teach Latin.

When it comes to primary and secondary education (pre-university) in the UK, few children are taught the subjunctive today. But then a significant number of English teachers in the UK are almost illiterate. It is hard to get accurate data on that, because it is a highly contentious issue.

I rarely look at secondary school textbooks, but I do look at "English as a Foreign Language" (EFL) textbooks. The terminology has changed substantially over the last 50 years, which makes your question more difficult. But the EFL books I have seen rarely spend much time dealing with the subjunctive.

[Added later]

John Lawler made an interesting comment below. I should make clear that I am not expressing an opinion on the merits; I am just making the factual point that usage varies substantially even with the UK.

Plenty of academics speaking and writing "perfect" English with RP accents (=Received Pronunciation, still used by the majority of those in senior government positions, the older universities, and in many of the traditional professions) would strongly attack the use of the subjunctive as a needless complexity. They will attack correct spelling, on the basis that in Shakespeare's day, there was no such thing.

It will be interesting to see what happens. Will the internet entrench "correct" English (used, to be pejorative) by the "Establishment", or will we see English fracturing into ever more dialects?

[Sorry, must be getting tired, it took 3 edits to fix those typos]

You're confusing traditional Latin grammar terminology with English grammar terminology,
and with modern linguistic terminology, as well.

Mood, Voice, and Tense were traditional inflectional categories of Latin verbs. I.e,
every verb in Latin was inflected (marked uniquely) for some mix of mood, voice, and tense.
Latin had six tenses (by a strange coincidence the same six you listed),
four moods (indicative, subjunctive, imperative, and interrogative),
and two voices (active and passive). That was Latin.

English has two tenses (Present and Past), no moods, and no voices.
In particular, English has no subjunctive mood, so you don't have to worry about it any more.

However, many other languages have rich inflectional systems, even richer than Latin.
Sanskrit and Greek both had a Middle Voice as well as Active and Passive, for instance,
and an Optative Mood (used for things one wishes and hopes for), and Sanskrit also had a Benedictive Mood (used for blessings).

And that's just Indo-European. There are lots of other ways to organize these matters.

Subjunctive is a mood. The three moods are indicative, subjunctive, and imperative.

By infinitive, did you mean the forms "to walk" or "to run?"

I think some get confused, as in your comment about being "written indicatively" about what the subjunctive is. It expresses uncertainty. It often looks the same as some indicative past tense forms of the same verb, but it is not.

Just to complete your initial statement of "Any verb has a mood, a voice and a tense," verbs also have a person (1st, 2nd, 3rd) and number (singular or plural).

I'm no linguist, so I'll stick with what I hope is common sense (and makes sense).

It seems to me the subjunctive is most easily put into context by looking at other European languages where it's application (the expression of doubt) is made explicit in the use of dedicated verb conjugation / participle pairings. For example:

English: "Had I known": German / Spanish

Seen in this way, the subjunctive can be viewed as the adornment of specific temporal constructs with mood 'candy'. In english, the candy is missing.

In both examples, the resulting (foreign language) expressions are distinctive and easily recognised. My impression is, incidentally, that in these languages, the subjunctive is also put to more frequent use.

I see no opposite to 'subjunctive' as a linguistic term, but rather the simple negation of an expression in the subjunctive, as in "Had I not known".