Learn Dutch: Ik was boos, maar ik beloof voortaan minder te klagen. - I was angry, but I promise to complain less from now on.

The verb beloven often takes a full subordinate clause, like dat ik voortaan minder zal klagen. Because the subject making the promise (ik) is the exact same person who will do the complaining, you can drop the second ik and the auxiliary verb, replacing them with a shortened clause using te + infinitive (te klagen).

What dictates the word order of voortaan minder in the second half of the sentence?

In a shortened clause with te, the te + infinitive combination (te klagen) acts like the verbs at the end of a regular subordinate clause. It gets pushed to the very end. Any adverbs or objects, such as the time word voortaan and the adverb minder, must be placed before te.

Could I use vanaf nu instead of voortaan?

Yes, they are mostly interchangeable here. voortaan is a single adverb meaning from now on or henceforth and is perfect for stating a new ongoing habit or rule. vanaf nu means the same thing but puts slightly more emphasis on this exact moment being the starting point of the change.

Is it normal to switch from the past tense (was) to the present tense (beloof) like this?