There is nothing wrong with translating words that are actually in the French sentence if it is a very common, perfectly grammatical way to express it in English.
Duo should be encouraging the practice not discouraging it. They should be encouraging it because it will help students deal with the absolute requirement for those words that are not always present when translating from English to French.
I have added "some" here but be aware that the plural of a noun in English is usually accomplished by adding an "s".
- un livre, des livres = a book, books (the plural of "a book" is not "some books"--just "books")
- un homme, des hommes = a man, men (the plural of "a man" is not "some books"--just "men")
There is no difference between "they buy" and "they are buying". You probably said "some" which was previously not included. Although it is technically not even correct here, it is not accepted. Many learners have been taught to translate "des" as "some". But be aware that the plural of "a book" is "books", not "some books". In Duo's early days, "des" was almost always translated as "some" in an effort to ease learners into the idea of translating every word. In fact, English has no equivalent for the plural of "a/an". This use of "some" is almost always ignored in English.
Yes, they are pronounced differently. Note that when an adjective precedes a plural noun, the normal "des" is changed to "de". These two words sound different so you may need to tune your hearing for it.
- un haricot, des haricots = a bean, beans
- un long haricot, de longs haricots = a long bean, long beans
So bloody confusing, are these yellow bean that are long or are they called long beans that are yellow. From the translation I know they are yellow beans that are long, seems when I apply that to translated in french, it becomes long beans that are yellow. Subtle difference.
Because "être en train de" places far more emphasis on the fact that it is happening right now than English's "are buying".
In English the -ing form is the most usual form of the present tense for things that are not habitual. (It's called the continuous present.) French does not have this form.
So "ils achètent" can be translated equally as either "they buy" or "they are buying".
Adjectives that come after the noun in French classify the noun. This Duo sentence says that beans are a type called jaunes/yellow. They are the product of a crop of vegetables called yellow beans. Those yellow beans have inherent qualities...long, short, fat, narrow etc. Inherent qualities go in front of the noun.
So we know that the the beans are of the yellow type and they happen to be long. Now how to express that in English. The English part of the Duo example does not have a comma between long and yellow. That means that long does not modify the beans but instead modifies yellow.
So both the French and the English tells us that there are a bunch of the yellow kind of beans and that they happen to be long. Your English sentence with yellow modifying long means that beans are of the long variety and happen to be yellow.
If you ordered a bunch of yellow beans, believing them to always be short and got instead a bunch of very long beans most of which were yellow, you would be upset because there is, in fact, a difference. Ok. Maybe you wouldn't be upset except for saying....damn that comma thing got me again! Or maybe more tolerantly......gee, that adjective in front of or after the noun in French is tricky to apply when you are just talking about stuff.