one must be careful
'hay que' is impersonal (no specific subject). Followed by infinitive.
You have to be careful of the ice on the road.
—Is the neighbourhood dangerous at night? —You do have to be careful, yes.
You have to be careful when handling chemicals.
You have to be careful of the ice on the road.
—Is the neighbourhood dangerous at night? —You do have to be careful, yes.
You have to be careful when handling chemicals.