Question

When do we use adj. before noun and when do we use noun before noun to make compound noun? E.g: We say: theory lesson or theoretical lesson

Answer 1

We use the adjective before the noun always, and the adjective after the noun when there's a verb in the middle.

I.e: "This is a pretty flower" // "This flower is beautiful".

Answer 2

adjectives describe nouns - nouns are words used to name a person, animal, place, thing, and abstract idea.

- you can use an adjective before a noun like this:

She was wearing a beautiful dress for her cousin's wedding.

- you can use an adjective after a noun like this:

The dress she wore for her cousin's wedding was beautiful.

as for your questions whether is should be theory lesson or theoretical lesson - it should be theorical lesson since theory is a noun, not an adjective. Theoretical is adjective that describes that the lesson is theoretical in nature.

just to keep it simple:

remember: what is being described (noun) and how the noun is described (adjective)