Had Tom zich maar niet versproken.

Because this is not a normal statement. It is a wish/regret construction, similar to English If only Tom hadn’t misspoken.

A regular past perfect statement would be:

Tom had zich niet versproken.
= Tom had not misspoken.

But Had Tom zich maar niet versproken. expresses an unreal wish about the past: the speaker wishes that something had not happened.

So the fronted Had gives the sentence an emotional, exclamatory feel.

Here maar does not mean but.

In this kind of sentence, maar is a particle that strengthens the wish or regret. It is very common in Dutch expressions like:

Had ik maar geluisterd.
= If only I had listened.

So in Had Tom zich maar niet versproken, maar helps create the meaning if only or I wish.

Because the verb is zich verspreken, a reflexive verb.

Zich verspreken means to misspeak, to make a slip of the tongue, or to let something slip by accident.

So you do not normally say just Tom had versproken for this meaning. You say:

Tom had zich versproken.

The reflexive pronoun changes with the subject:

• ik verspreek me
• jij verspreekt je
• hij verspreekt zich
• wij verspreken ons

Since the subject is Tom, the correct reflexive form is zich.

Versproken is the past participle of zich verspreken.

The full verbal form here is:

had ... versproken

That is the past perfect.

So grammatically, the sentence contains:

• had = auxiliary verb
• versproken = past participle

Together they mean something like had misspoken.

Because the verb begins with the inseparable prefix ver-.

In Dutch, verbs with inseparable prefixes such as be-, ge-, her-, er-, ont-, and usually ver- do not take ge- in the past participle.

So:

• verkopen → verkocht
• vertellen → verteld
• zich verspreken → versproken

That is why geversproken is not correct.

Because in Dutch clause structure, niet usually comes before the final participle or infinitive in this kind of sentence.

So:

Had Tom zich maar niet versproken.

The negation applies to the whole verbal idea zich versproken hebben.

Compare:

• Tom had zich niet versproken.
• Had Tom zich maar niet versproken.

In both cases, niet appears before the past participle versproken.

The form had ... versproken is the past perfect.

It refers to something that happened before another point in the past, but in this kind of wish sentence it is mainly used to express regret about a past event.

So the idea is:

• Tom did misspeak
• the speaker wishes that had not happened

That is why Dutch uses had here rather than heeft.

Compare:

• Tom heeft zich versproken.
  = Tom has misspoken / Tom misspoke.

• Had Tom zich maar niet versproken.
  = If only Tom had not misspoken.

Yes. In Dutch, this can stand on its own as a complete exclamation of regret or wish.

It may feel elliptical to an English speaker because English often uses if only. Dutch can express the same idea more directly with this inverted form:

Had Tom zich maar niet versproken.

So although it looks like part of a conditional sentence, it works perfectly well by itself.

Not with the same meaning.

Tom had zich maar niet versproken sounds wrong or at least very unnatural if you want to express a wish/regret.

For the regret meaning, Dutch normally uses the inverted form:

Had Tom zich maar niet versproken.

That inversion is an important signal that this is not an ordinary factual statement, but an emotional wish about the past.

A good way to think of it is:

• Had = marks a past unreal wish
• Tom = subject
• zich verspreken = reflexive verb
• maar = wish/regret particle
• niet = negation
• versproken = past participle at the end

So the pattern is roughly:

Had + subject + reflexive pronoun + maar + niet + past participle

This pattern is very useful to recognize in Dutch, especially in sentences expressing regret such as:

Had ik maar meer tijd gehad.
Was hij maar eerder gekomen.
Had ze het maar gezegd.