I was just wondering how can you use "there is" followed by a word that is in plural, e.g
"There's cupcakes in the fridge"
"There is a couple of things we can do --"
Is it just slang?
Existential "there" is an unusual subject; it has no inherent number, but takes on the number of the displaced subject:
"There were some keys near the safe". (plural with "were")
"There was a nurse present". (singular with "was")
It’s comparable to the relative pronouns "which" and "who", which take on the number of their antecedent ("the guys who were talking" vs "the guy who was talking"). However, in informal style, especially in present tense declaratives with reduced "is", many speakers treat "there" as always singular: they say "There’s a few problems" instead of "There are a few problems". Prescriptivists disapprove, but the usage is too well established to be treated as an occasional slip.