Learn German: Wenn man hier wandert, sieht man einen schönen Wasserfall. - When one hikes here, one sees a beautiful waterfall.

Because the first part of the sentence is a subordinate clause starting with Wenn. This pushes its verb, wandert, to the very end of that clause. The main clause then begins immediately after the comma. In German, the verb must be in the second position of its clause. Since the entire Wenn clause counts as position one, the main clause verb sieht must come first in its own clause. This creates the classic verb-comma-verb pattern.

Could I use Als or Wann instead of Wenn here to mean when?

No, Wenn is the only correct choice here. Use wann only in questions. Use als for a single event in the past. Because this sentence describes a general, repeating condition in the present, you must use wenn.

How does the pronoun man work in this sentence?

The word man is an impersonal pronoun meaning one, you in a general sense, or people. It is always treated as a third-person singular subject, which is why the verbs take the -t ending (wandert, sieht), exactly as they would for er, sie, or es.

Why does the adjective take an -en ending in einen schönen Wasserfall?