Translation:My wife often talks to the plants.
So my answer was "My wife often speaks WITH the plants." Got it correct. However, another correct solution is "My wife often speaks TO the plants."
I didn't think much of it at first, but these are actually different things. [To] is one-directional. [With], on the other hand, is used when there is a conversation. In this case, I don't think the plants are talking back (at least without the help of psychoactive drugs).
Is [mit] the correct word to use here or would it be some other more literal translation of [to] such as [zu/an]?
This is more contextual than grammatical. Basically "sprechen mit" means "speak with" so in general it is always speaking with someone/something. You could also think of it this way, you may have a conversation with someone who is completely reluctantly to respond, but you can still consider you speaking with him/her. =)
I'm not sure it's what caused your answer to be marked as incorrect, but I would expect a comma after often when it starts the sentence. Especially since there is generally a slight pause after saying it before continuing with the rest of the sentence.
That being said, they don't usually mark answers as incorrect for punctuation so probably best to report it.
You can use a comma after "often," but it isn't necessary because it's such a short phrase.
I'm not sure the length of the sentence has any bearing as to whether a comma is appropriate/required or not. Otherwise, you wouldn't have sentences like:
"Finally, he came."
"Yes, I do."
Plus, one of the places for commas is after introductory adverbs, which often is.
I'd argue the reason for moving the adverb to the front is generally to create a rhetorical device and not just an arbitrary structure change. As such, it should probably contain the comma.
"My wife often talks to the plants" and "Often my wife talks to the plants" are equivalent sentences.
Don't forget "My wife talks to the plants often." Adverbs can go at the end as well.
I don't think "often" can go together with "is speaking" or any continuous form of a verb for that matter, since it never indicates a continuous action. You could however say "my wife is always speaking to the plants", which would mean you are annoyed with it, but this is not a good translation for the German sentence.
"My wife often speaks to (the) plants" was accepted by the way. :)
As far as I know, it shouldn't be wrong. I can never seem to get the grasp of the difference between (in this case) "she talks" and "she is talking".
"often" refers to a series of discrete events and therefore necessarily refers to events in a time frame other than this present moment, whereas the present continuous refers to a single activity which is continuous and continues right up to, and including, the present moment.
Besides that, this sentence was probably in the "frequency" section and they were introducing oft as a frequency word.
Germans could say the same thing if they were translating "My wife often talks to the plants." and they only wrote "Meine Frau spricht mit den Pflanzen." I would expect it to be counted wrong as the original sentence had a frequency word.
"Usually" and "often" are not completely synonymous. Let's try another example, to clarify. Let's say you go out for dinner every Friday night.
Maybe once a month, you go to the corner bistro: that's enough to say you often go to the corner bistro.
The other three Fridays in the month (or four, if there are five Fridays that month) you go to McWallabee's Infamous Steak Place.
If that's the case, you often go to the corner bistro, but you usually go to McWallabee's.