Translation:Who is the strongest man in the Netherlands?
I still don't get when to use "de" and "het" and words like "sterkst" and "sterkste". Because trein is also a de-word, but another sentence said "Welke trein is het snelst?" Or is it the way the words in the sentence are arranged? So "Welke man is het sterkst" and "Wat is de snelste trein" are both correct?
There is a note about it in the lesson:
De jongen is het snelst. - The boy is the fastest.
Note that even though jongen is a de-word, the superlative in that last sentence becomes het snelst. The reason is that it is a predicate adjective (it comes after the noun), which does not reflect the gender (de/het) of the noun. However, it is also allowed to say:
De jongen is de snelste. - The boy is the fastest. In this case it is implied that the noun jongen is repeated after the superlative: "De jongen is de snelste (jongen)."
But precisely here, the superlative works as an adjective and is used with a "de" noun, so you follow the adjectives rules: de/het + adjective + e - "de sterkste man".