Learn Dutch: Wij zamelen veel kleding in voor de stichting. - We are collecting a lot of clothing for the foundation.

It is the prefix of the separable verb inzamelen (to collect). In a standard main clause, the verb splits: the conjugated part (zamelen) takes the second position, and the prefix (in) moves toward the end of the clause.

How would this sentence change if we wanted to say 'all the clothing' instead of 'a lot of clothing'?

You would say al de kleding. As you are learning in this lesson, when you refer to a specific, definite mass noun that is a de-word (like kleding), you use al de. If it were a general plural, you would use alle. Here, with the quantifier veel (a lot), it simply stays veel kleding.

Is kleding considered singular or plural in Dutch?

Kleding (clothing) is a singular, uncountable noun, exactly like in English. If you want to count specific items, you have to use a word like kledingstukken (pieces of clothing).

Could the separable prefix in go at the very end of the sentence?

Yes! You can absolutely say Wij zamelen veel kleding voor de stichting in. It is very common in Dutch to place a prepositional phrase like voor de stichting (for the foundation) after the separable prefix, treating it as an extra detail added to the core sentence.