Can someone tell me how the following two sentences differ?
Das ist doch Marlene!
Das ist Marlene!
This doch is either a modal particle or an adverb.
Das ist doch Marlene! (particle, surprised about Marlene's presence)
Das ist doch Marlene! (adverb, indicating someone identified the person wrong before)
The difference between the two can be told by the stress.
Without the doch the sentence is a plain identification.
Das ist nicht Helene. Das ist Marlene.