Learn Dutch: Ik zie hem knikkend weggaan. - I see him walk away, nodding.

To make the present participle (words often ending in "-ing" in English that describe an ongoing action), you just take the full infinitive of the verb and add -d. So, knikken (to nod) becomes knikkend (nodding). Here, it acts as an adverb to describe how he is walking away.

Why is it knikkend and not knikkende?

In this sentence, knikkend is used as an adverb because it describes the verb weggaan (how he is leaving). When present participles are used as adverbs, they never get an -e at the end. You only add the -e when it's an adjective placed directly in front of a noun (like de knikkende man - the nodding man).

Why is the verb weggaan at the very end of the sentence?

When you use a verb of the senses like zien (to see) or horen (to hear) alongside another action, that second action gets pushed to the end of the sentence as a full infinitive. This is very similar to English, where we say "I see him walk away" instead of "I see him walks away".

Isn't weggaan a separable verb? Why aren't weg and gaan separated here?