This is advisable. Do not permit our competitors to know our plans.
This is advisable that do not permit our competitors to know our plans.
This is imperative. Get your passport renewed before you leave the country.
This is imperative that get your passport renewed before you leave the country.
You cannot use the second and fourth forms. If you wish to use the subjunctive to express a command with no commander (person giving the command), you can only ever use It as the subject of the matrix (outer) clause.
Additionally, a clause embedded by that must be well-formed, subjunctive (which is essentially identical to the declarative in modern English) clauses: it must have a subject, a finite verb, and objects/obliques if necessitated by the verb.
Although "you" may mean the person you are writing for/to or addressing, it can stand for "one", and is probably more idiomatic than "one" in modern English. Since "you" is number-agnostic, it is safe to use.
I would also consider that the first and third sentences are unusual in their Theme-New structure - you refer to this but you introduce this in the next sentence. While cataphoric reference is perfectly legitimate, the information flow is unusual, and particularly so in the first example.
I would write those sentences as:
Do not permit our competitors to know our plans. This is advisable.
It is advisable that you do not permit our competitors to know our plans.
Get your passport renewed before you leave the country. This is imperative.
It is imperative that you get your passport renewed before you leave the country.
The comments resulting from this Q&A have raised important points that are worth raising as an addendum:
- In Modern English, the subjunctive isn't really as productive as it is in other languages - the only verb that obligatorily conjugates differently in such constructions is be, and even this is phasing out.
All other verbs can take the infinitive (minus to), but it is perfectly acceptable to use the declarative in these senses. (For instance, either We demand that Tom hire her immediately or We demand that Tom hires her immediately could be said,)
- These "subjunctive" clauses are everyday, straightforward declarative clauses - they have a subject and finite verb like any other.
- @StoneyB has said that because these often express mandates, it can be useful to think of the as imperative clauses. I would argue this is true only to the extent that the meaning can be imperative. Imperative clauses, however, do not generally have finite verbs or subjects (with the exception of let's and third-person subjects, as in Chocolate lovers rejoice!).
- In short: it may be easier to think of these constructions as merely two clauses - clause + clause - since they don't require any special treatment. Subjunctive makes things confusing, and is a relic of the past (or analogised from other languages), and has little or no place in today's English.