Learn French: Ce tunnel immense contiendrait beaucoup de choses. - This immense tunnel reportedly contains many things.

You've spotted the "journalistic conditional"! In French, reporters and writers use the conditional tense to share information that hasn't been officially confirmed yet. While contiendrait literally means "would contain," in English we naturally express this doubt by saying "reportedly contains" or "is said to contain."

How do I form the conditional for a verb like contenir?

Contenir (to contain) follows the exact same irregular pattern as venir (to come) and tenir (to hold). Instead of using the whole infinitive as the base, the stem changes to contiendrai-. Then you add the standard conditional endings (which are identical to the imparfait endings). For ce tunnel (it), the ending is -t, giving us contiendrait.

Since choses is plural, shouldn't it be beaucoup des choses?

This is a very common trap! After expressions of quantity like beaucoup (a lot / many), peu (few), or trop (too much), you almost always use a bare de or d', regardless of whether the noun is plural. You would only use des if you were specifying which things, like beaucoup des choses que tu as dites (a lot of the things that you said).

I learned that size adjectives go before the noun. Why is it ?