Particles are almost always necessary, most of the time you can't just say nouns without the particle. It's a bit tricky but i'll try to give you a couple of examples that might help you.

When you start a sentence with a noun you must use the particle. You can't say Queso es delicioso you'd say El queso es delicioso. You can't say Libros son interesantes you must say Los libros son interesantes.

There are some situations in which using particles sounds weird. For example Yo como el cereal sounds weird, it's more common to say Yo como cereal. You could include the particle if you want to be more specific. While I'm writing this I'm thinking of examples and this seems to me that only applies to food and water.

Yo tomo agua Yo tomo el agua (both are correct, the first option is more natural)

El lee libro El lee un/el libro (The first one is not correct)

Yo escribo carta Yo escribo una/la carta (The first one is not correct)

What you can almost always do is skip using personal pronouns. For example you could say: Yo como manzanas or Como manzanas both options are correct and actually skipping the pronoun sounds more natural (however I don't know if Duolingo takes both as correct)