'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 &)
I believe it has something to do with "Architekt" being in the accusative case. Sometimes, nouns may change when they are accusative. Such as how "Junge" changes to "Jungen".
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.
And to das Herz which is neuter, but also has genitive/dative des/dem Herzen and plural die Herzen, der Herzen, den Herzen.
(But singular accusative das Herz, because neuter always has accusative = nominative.)