Had ik de gastenlijst maar gecontroleerd.

Because this is a special Dutch pattern used to express a wish or regret: Had ik maar ...

It does not work like a normal statement. A normal past perfect statement would be:

• Ik had de gastenlijst gecontroleerd. = I had checked the guest list.

But when Dutch wants to say If only I had checked the guest list, it often puts the finite verb first:

• Had ik maar de gastenlijst gecontroleerd.

So the verb-first order here signals an exclamatory or wish-like meaning, not a regular statement.

No. In this sentence, maar does not mean but.

In the pattern Had ik maar ..., maar helps express a strong wish or regret, like:

• if only
• I wish
• I should have

So here maar is part of the idiomatic regret structure. It is emotional rather than contrastive.

Compare:

• Ik wil koffie, maar geen thee. = I want coffee, but not tea.
  Here maar really means but.

But in:

• Had ik maar opgelet. = If only I had paid attention.

it is a marker of regret.

Yes, that is the closest natural English equivalent.

The sentence expresses:

• a wish about the past
• regret
• something that did not happen, but should have happened

So Had ik de gastenlijst maar gecontroleerd means roughly:

• If only I had checked the guest list.
• I wish I had checked the guest list.

Because Dutch perfect tenses usually place the past participle at the end of the clause.

This sentence uses:

• had = auxiliary verb
• gecontroleerd = past participle of controleren

So the structure is:

• had
  • subject + other parts + gecontroleerd

That is very normal in Dutch.

Compare:

• Ik had hem gezien. = I had seen him.
• Had ik maar beter geluisterd. = If only I had listened better.

It is the past perfect (also called the pluperfect).

The form is:

• past tense of hebben or zijn
• plus a past participle

Here:

• had = past tense of hebben
• gecontroleerd = past participle

So:

• ik had gecontroleerd = I had checked

In this sentence, that tense is used in a regret construction about the past.

Because the verb controleren takes hebben as its auxiliary in perfect tenses.

So:

• Ik heb de gastenlijst gecontroleerd. = I have checked the guest list.
• Ik had de gastenlijst gecontroleerd. = I had checked the guest list.

Some Dutch verbs use zijn, especially verbs of movement or change of state, but controleren does not.

Gastenlijst is a compound noun:

• gast = guest
• gasten = guests
• lijst = list

So:

• gastenlijst = guest list

Dutch makes compounds very freely, and they are usually written as one word.

Because gastenlijst is a de-word.

In Dutch, every noun takes either de or het. There is not always a logical rule you can see from the meaning alone, so this often has to be learned with the noun.

So:

• de gastenlijst
• not het gastenlijst

Yes, Had ik maar de gastenlijst gecontroleerd is very common and often sounds more neutral.

Your sentence:

• Had ik de gastenlijst maar gecontroleerd

is also possible, but placing de gastenlijst before maar can give that object a bit more focus.

So both can work, but learners will very often first meet the pattern as:

• Had ik maar ...

For example:

• Had ik maar geluisterd.
• Had ik maar beter opgelet.
• Had ik maar de gastenlijst gecontroleerd.

Not in normal modern English.

A word-for-word translation like that is not natural English. The natural translation is:

• If only I had checked the guest list or
• I wish I had checked the guest list

This is a good example of where you should learn the whole Dutch pattern Had ik maar ... as an expression, not translate each word separately.

Yes. Very strongly.

The sentence expresses regret about a past situation that did not happen. So the speaker is implying:

• I did not check the guest list.
• That was a mistake.
• I now regret it.

That is why the sentence has a strong counterfactual meaning: the speaker wishes the past had been different.

No, that would not be the normal way to express this meaning.

For regret about the past, Dutch usually uses the past perfect pattern:

• Had ik maar de gastenlijst gecontroleerd.

Using a simple past like controleerde would not naturally express If only I had checked...

So if you want past regret, learn this formula:

• Had + subject + maar + past participle

The infinitive is:

• controleren = to check

The past participle is:

• gecontroleerd = checked

This is a regular weak verb. Very roughly:

• infinitive: controleren
• stem: controleer
• past participle: gecontroleerd

You will often see it in perfect forms such as:

• Ik heb het gecontroleerd.
• Ik had het gecontroleerd.

Yes, absolutely.

Dutch often uses this kind of short exclamatory sentence by itself:

• Had ik maar geluisterd.
• Was ik maar thuisgebleven.
• Had ik de gastenlijst maar gecontroleerd.

It may feel as if English wants an explicit If only..., but in Dutch this is already a complete and natural sentence expressing regret.