Hi Ashlyn

Interesting question. Some adjectives in English are used in the -ing form as a present participle. This is the case of "hiding" in this sentence: I'm hiding. 

In Spanish this takes the form of "escondido" because "escondiendo" is not an adjective for us. It can only be part of a continuous tense, e.g. "Los niños se están escondiendo en los arbustos." (The boys are hiding in the bushes.) 

In English you can also use the adjective "hidden", which would correspond with the Spanish "escondido" and you can say something like:

They couldn't find the children because they were completely hidden in the bushes.

No encontraban a los niños porque estaban completamente escondidos en los arbustos.

There are other adjectives in English using the -ing form, for example:

We didn't want to wake up the sleeping children.

In Spanish this adjective "sleeping" would be "dormidos", not "durmiendo". 

Or:

She is lying on the sofa.

Ella está tumbada en el sofa. (not tumbando)

I hope this clarified it. 

Saludos cordiales