That's very confusing if you're a native German speaker ("Tomaten" is the plural of "Tomate" in German. Hearing the same word for tomatoes as word for the tomato is challenging :))
Is there a reason that using "the" makes some words end in -et and others in -en? Or is it something you just need to memorize like when to use ett or en?
Very roughly, ett-words get -et and en-words get -en. There are more on this in the Tips & Notes.