When is the article used?
In my test it asked me how to say "they eat strawberries" so i translated it as "ellos comen las fresas". It proved me wrong by crossing out the article. In other cases the articles are used (e.g. ella come un tomate). Can someone explain this to me?
Spanish is pretty similar to English when it comes to articles. When in English you use "the", you will most likely use the corresponding article in Spanish. When English doesn't use an article, most times Spanish won't either. This happens when the noun is plural and the singular version used the indefinite article "a(n)":
I ate the strawberry -> I ate the strawberries. I ate a strawberry -> I ate strawberries.
The closest English has to a plural indefinite article is the word "some". You could say "Ella come unas fresas" but, like using "some strawberries", it sounds like you are refering to a specific group or amount of strawberries.