Learn Finnish: Me olemme alennusmyynnissä, ja alennukset ovat suuria. - We are at the clearance sale, and the discounts are big.

When describing a plural subject with the verb olla, the predicative adjective is generally in the partitive plural (suuria) to express a quality. Using the nominative plural (suuret) would mean "the big ones", acting as an identifier for a specific, restricted group (e.g., Nämä alennukset ovat ne suuret).

How would I replace suuria to say the discounts are "double" or "twofold" using a multiplicative adjective from this lesson?

You would use the adjective kaksinkertainen. Following the exact same predicative rule as suuria, it must be put into the partitive plural to describe the plural subject: ...ja alennukset ovat kaksinkertaisia.

Why do we say alennusmyynnissä (inessive case) instead of alennusmyynnillä (adessive case) when a sale is an event?

While many events and activities take the adessive case (like kurssilla or luennolla), alennusmyynti takes the inessive case (-ssä). You can think of it as physically being "inside" the store where the discount sale is happening.

How are the parts of the compound word alennusmyynti derived?