Here's a trick that might help: "is", and other forms of the verb To Be, is a linking verb. The only adverbs that linking verbs co-occur with are those that express time and location, like this,
The baby is
in bed. (Where? In bed. (adverb))
The baby was in bed
at 6 p.m. (When? At 6 p.m. (adverb))
The baby was
asleep. (adjective)