Grammar: Relative Pronoun at "Beginning" of Sentence

In both classical and medieval Latin, it is extremely common to "begin" a sentence with a relative pronoun. You will find it much easier to mentally replace that relative pronoun with a demonstrative pronoun, giving yourself a kind of "mental break" between the clauses:

Marcus loquebatur de rebus multis.
Cui respondit Camilla: "Tace, o stulte!"

is the same as:

Marcus loquebatur de rebus multis.
Ei respondit Camilla: "Tace, o stulte!"
(or: Marco respondit camilla: "Tace, o stulte!"