'Die Familie hat einen Architekten' why is it Architekten and not Architekt?
Here are two good posts about weak nouns:
You can look up all the inflections for a word on this website:
http://canoo.net/services/Controller?input=Architekten & service=inflection
(duolingo won't allow the & symbol in a link for some reason, so cut and paste that and remove spaces next to &)
Architekt is subject to what is known as "N-Declension", which means that in all cases except for singular nominative it has an -en ending. This only happens to certain masculine nouns.