Questions & Answers about Het plan was ambitieus, maar ik beloof te slagen.
How does "te slagen" replace a "dat" clause in this sentence?
The phrase te slagen is a shortened subclause (beknopte bijzin). You could also say ik beloof dat ik zal slagen (I promise that I will succeed). Because the subject of the main clause (ik) is the exact same as the implied subject of the subclause, we can drop the second ik and simply use te + the infinitive slagen.
Why isn't there an "om" here, like "ik beloof om te slagen"?
While om ... te is very common in Dutch, certain verbs like beloven (to promise), proberen (to try), and weigeren (to refuse) often connect directly to te + infinitive without using om. Adding om isn't strictly wrong in spoken Dutch, but leaving it out is the standard, more elegant structure with these specific verbs.
I usually see "slagen" used for passing an exam. Does it work differently here?
It is the exact same verb! While slagen voor een examen means "to pass an exam", the core meaning of slagen is simply "to succeed". In this sentence, the speaker is promising to succeed at executing the ambitious plan.
The first half is in the past tense (was), but the second half is in the present (beloof). Is this tense mixing normal?