Once again Duolingo is giving me an English lesson!!! "They are preparing vegetables" and "they are preparing THE vegetables" are EXACTLY THE SAME in ENGLISH. "Vegetables" is an uncountable noun and therefore means the same whether you use "some" or "the". This is just petty, nitpicking and NOT helpful to me as regards my ability to learn French. I have limited time, and if I keep losing hearts on such petty things, I then have to start another exercise from the beginning. Grrrr
Here is the problem. There is a difference in English between I am preparing vegetables and I am preparing the vegetables. There is even a third possibility which will become more clear if I change the verb.
Question: What do you do for a living?
I buy vegetables = I buy all vegetables, every vegetable I can get my hands on, all the vegetables in the store, all the vegetables on the market, all the vegetables in the world if I can get control of them.
I buy vegetables = I buy some vegetables, not all but just some, just ones fit for human consumption, only those that I can physically manage, just those that I can afford,
I buy the vegetables = I buy those vegetables right there, those vegetables that we know about, those vegetables that I'm known for using, those vegetables that we were just talking about.
Each of those answers has a different meaning. In the case of the first two, English speakers routinely drop the article and leave it up to the listener to figure out which is the appropriate meaning. They do this to the point that they are sometimes unaware that they are not disclosing what was intended by their remark.
English speakers do this because:
Context usually makes it obvious which meaning applies.
Even if context doesn't indicate the intent, it's usually not important anyway
If context doesn't help and it is important to the listener, well, they can just ask what was meant.
Unfortunately for English speakers, French usually requires a modifier for most nouns and very often it's an article.
le/la = the (specific) that one right there, that one that we know about
du/ de la = of the = some (usually), some but not all, unspecified, general but limited
le/la = the (general) all wine, all examples of wine, the idea of wine.
The third one is tricky for English speakers because there is no comparable article in English. French speakers have assigned a dual role to le/la where they assign it the job of general article. Only context can tell you which meaning applies to le/la. Very often the verb provides context.
When an English speaker says I like wine most listeners will assume he means he likes all wine. When an English speaker says I like to drink wine the listener will assume the speaker means he likes to drink some wine because he can't drink all the wine in the world. In English, the distinction is left unstated. In French it is not.
Duo wants to make sure you understand the difference in English because there is an even greater requirement to understand it in French if you are going to translate it accurately.