"A cat is lying on the bed."
Translation:En katt ligger på sengen.
This course doesn't teach the indefinite singular article anyways, just in one or two courses I believe, as it's not commonly used in Norwegian Bokmål, so it's nothing to worry about.
There are three ways to inflect 'katt' because of this:
(1) en katt - katten
(2) en katt - katta
(3) ei katt - katta
Note that 'ei katt - katten' is not accepted. This course would teach (2), as it's common to use the definite feminine form, but rare to use the feminine indefinite singular article.
"Ei katte" is not nice. If you love cats, forget about "ei katte, katta.
"En katt" is the words to remember.
If you want to say: "I have a female cat", you say: "Jeg har en hunkatt."
I would answer: " Så fint !! Jeg har tre kastrerte hankatter."
( actually I had four last week, but one was very sick with diabetes so he is no longer here).