"I do not understand him at all."
Translation:Vůbec ho nechápu.
I had the same question. It relates to using "ho" which likes to be in the "second position". Do adverbs, in this case vubec, command the first position when they begin the sentence? FYI, my answer was "Vubec ja ho nechapu." which was incorrect. The correct solution was "Vubec ho nechapu."
if adverbs show up in the first position, they do count.
what does not count as the first constituent is conjunctions a, i, ale.
your comment about prepositions not counting suggests some confusion. prepositions cannot form a complete constituent without their noun phrase, and that is why you can't plop the clitics right after a clause-initial preposition.
the second position does not automatically mean the second word:
- Na něho se nedívám.
- ??Na se něho nedívám.
because one cannot separate the preposition from its noun phrase.
and it is
- Na toho velkého psa se nedívám.
- ??Na toho se velkého psa nedívám.
- ??Na toho velkého se psa nedívám.
because one cannot split up the noun phrase either.