Learn Dutch: Nou, ik weet het niet.

In Dutch, the verb weten (to know) usually needs a direct object. If you aren't mentioning the specific fact you don't know, you use het (it) as a placeholder. A Dutch speaker literally says "I know it not."

Why does niet come at the very end of the sentence?

In Dutch, the word niet (not) comes after a specific object. Here, the specific object is the word het (it). So the order is subject (ik), verb (weet), object (het), and finally the negation (niet).

Can I use goed instead of nou to say "well"?

No. English uses "well" in two ways: as a description of doing something nicely (like "I sleep well") and as a conversational filler word (like "Well, let me think"). In Dutch, you use goed for the first meaning, but nou for the filler word used here.

What is the full dictionary form of weet?

The full verb (the infinitive) is weten (to know). It is slightly irregular. For all the singular forms (I, you, he/she/it), the verb drops the 'en' and is just weet, which is why we say ik weet.