Q:

singular or plural verbs for compound nouns and modifiers

I am confused why the compound noun has the singular verb "HAS" in the modifier clause "which has won awards..." and the plural verb "HAVE" as the main verb.
Should I use plural or singular verb as the verb in the main and modifier clause?
A national supermarket chain and a locally owned bakery, which HAS won awards for its crosissants, HAVE seen a significant decrease in the revenue since the biggest employer in the area shut down.

A:

Read it this way:

A national supermarket chain and a locally owned bakery (which HAS won awards for its crosissants[sic]) HAVE seen a significant decrease in the revenue since the biggest employer in the area shut down.

The has applies to a bakery and have to both:  the supermarket chain and bakery. 
Check restrictive and non-restrictive clauses here. 

