Does subjunctive structure always use 'that' word in the begining? How exactly it works?

Question

I have an idea about it but not utterly sure how does it work? I read it in books but it's still confusing; for instance, is it mandatory to be preceded always by the word that?

Is there any rules on the base form on the sentence?

Answer 1

This is too broad a question to answer. With multiple sources, let me jot down what you want.

There are three moods of English verbs: indicative, imperative, and subjunctive.

The commonest is indicative mood that shows a fact/an opinion or forms a question

I am going to market. Do you want to come with me?

The imperative mood commands, directs, requests

Go to the market (to bring bread) -it takes off the subject of the sentence

The most complicated is subjunctive mood that shows a wish, suggestion, and attitude. It uses I were and such distinctive forms of the verbs.

If I were you, I would go to market (in spite of heavy rain).

Making the Subjunctive Mood

The present subjunctive takes the most general form of the verb irrespective of the subject

The priest advised that I pray daily.

The past subjunctive uses the same form what the past tense of the verb is.

I wish I were God; If I were rich, I could retire.

When the subjunctive is used, use the past subjunctive in contrary-to-fact clauses stating conditional statement. These statements generally takes if.

For present contrary-to-fact clauses, the past tense of the verb is used. And, use were in case of the verb is to be.

If I were in New York, I would witness every celebration happening on the Times Square.
I wish I were tall dark and handsome, but I am short, fair and ugly.

For past contrary-to-fact clauses, the past perfect of the verb (had + past participle) is used.

If Rocky had studied harder, he would not have been failed.

Use the present subjunctive in sentences expressing suggestions, needs, or requests. Such instances take verbs like ask, recommend, etc. These verbs often take that (your concern in question).

The teacher insisted that Rocky study hard to get good numbers.
The priest asks that each disciple pray four times in a day.

Good read here.