"He eats a tomato."

Translation: Han spiser en tomat.

[AmericanZues]

How do you tell the difference between et words and en words?

[NordOfTheSouth]

It is simply a matter of memorization. A rule of thumb is that animate objects use "en" and inanimate use "et", but that isn't always the case. I have read that Danes will be able to tell a native speaker from their proper use of en and et

[davidbakposada]

Why can't you use "tomaten"?

[Xneb]

En tomat = A tomato
Tomaten = The tomato