"They are washing the potatoes."
Translation:Sie waschen die Kartoffeln.
I know that Kartoffeln is plural
Gender is irrelevant in the plural. There's no "feminine plural" in modern German. Just "plural".
I doubt that Kartoffeln is accusitive
Why? It's the direct object of the verb waschen, so it's in the accusative case in this sentence.
this is why I believe that DEN is never used as a pronoun for either Kartoffel or Kartoffeln.
den is an article, not a pronoun.
And den is not only masculine accusative but also plural dative.
It's a possible translation, if you want to emphasise either the subject, the object, or the verb -- in English, you might use a cleft for this: "It's the potatoes that they are washing" or "It's they who are washing the potatoes" or "What they do to the potatoes is washing them".
You can tell the difference between "she" and "they"/"you" by the verb form, which ends in -t for "she" and in -en for "they"/"you".
For example, sie wäscht with -t for "she is washing", sie waschen with -en for "they are washing", and *Sie waschen" with -en and always with capital S for "you are washing".
In the middle of a sentence, you can tell the difference between "they" and "you" because "they" has lowercase sie but "you" has capitalised Sie. At the beginning of sentence, when sie/Sie is the first word of the sentence and is therefore always capitalised, only context lets you know whether the word means "they" or "you".
There is no preposition in this sentence.
die Kartoffeln is the direct object of the verb waschen, and it's in the accusative case.
Only masculine (singular) words have a separate form in German compared to the nominative, so the accusative plural looks exactly like the nominative plural, e.g. die Kartoffeln.
why is there no difference between "they wash" and "they are washing" for example.
Because German doesn't distinguish that grammatically.
Much as English doesn't distinguish "Tom did it (and I know that for a fact)" from "Tom did it (or so I heard)" grammatically, even though some languages go.
is there a way to differenciate these tenses?
No. And no need to, either.