Danish: to call someone
I translated "I usually call my secretary" to "jeg plejer at ringe min sekretær" and this was not accepted by duolingo. The correct sentence is "jeg plejer at ringe TIL min sekretær".
In Swedish, you can say either "jag brukar ringa min sekreterare" or "jag brukar ringa till min sekreterare". Is this not the case in Danish?
To say " I usually call you", one cannot say "jeg plejer at ringe dig", it must be "jeg plejer at ringe TIL dig" (directly translates as "I usually call to you") as you yourself noticed.
However, one can also say "jeg plejer at ringe dig OP" ("I usually call you up") which isn't at all as frequently used as the first sentence. Most adults and young people do not use "at ringe nogen op" ("to call someone up") a lot, if at all, in daily conversation. I'm not sure if Duolingo accepts it, but I think it should be valid.
EDIT: one of the most frequent uses of "At ringe op" is actually in the phrase "hvem har ringet dig op?" which means "who has called you?" and is a way of saying "who asked for your opinion?" It's considered pretty rude (for a swear-word free sentence) though.
Oh, I see! Thank you very much. It is always so interesting to find these cases where our languages differ.
Nope, it isn't.
"(at) ringe" can be used as follows
>subject< ringer (e.g. "klokken ringer" meaning "the bell rings")
>subject< ringer til >prep. object< (e.g. "jeg ringer til dig" meaning "I call you")