"Han er en vagt" means his profession. "Han er vagt" is either somthing he is currently doing. Or the start of a sentens explaing where he works as a guard i.e. "Han er vagt i smykkebutikken" he is a guard at the jewllerystore
Why is there a difference in pronunciation between fast and slow mode? Duolingo is consistently like this.
AFAIK, the fast mode is natural, and the slow mode is a set of separate words. Some words change a bit, when they are unstressed.
In one exercise there was no 'en' before profession, but here there is. Am I correct in assuming that a Dane would rather say this sentence as 'han er vagt'?