Deriving agreement variation with postverbal subjects

In a multitude of constructions across a variety of languages, it is possible to optionally realize verbal agreement with postverbal subjects for which agreement is obligatory when the subject appears proverbially. This is exemplified below in English.

(1) a. There {were/was} problems here. b. Problems {were/*was} here.

In novel research I analyze this agreement alternation as the output of a single set of syntactic operations that produce multiple Agree relations with a single probe, deriving the agreement optionality with postverbal subjects as well as ruling out ungrammatical agreement patterns and explaining variation in how postverbal subject is realized crosslinguistically.