People intuitively recognize passages of the Discourse Modes, although they are probably unaware of the linguistic basis for the differences between them. Each mode – Narrative, Description, Report, Information, Argument – introduces certain entities into the universe of discourse, with a related principle of discourse progression. The features have linguistic correlates of a temporal nature. In fact temporality in the larger sense is the key to the discourse modes. Temporal factors are woven into the fabric of a language and are part of our tacit knowledge of language structure.
I use the term "passage" for text segments that realize a discourse mode. Passages must be long enough to establish the linguistic features that determine a mode. Two sentences suffice to do this. Intuitions are particularly strong when there is a shift of mode. As an example, consider (1), the beginning of an article from the National Geographic. The discourse mode shifts twice: from Information to Narrative and back to Information. The title and paragraphing follow the original.
(1) Listening to Humpbacks
1 When a big whale dives, currents set in motion by the passage of so many
tons of flesh come eddying back up in a column that smooths the restless
surface of the sea. 2 Naturalists call this lingering spool of glassy water the
whale's footprint. 3 Out between the Hawaiian islands of Maui and Lanai,
Jim Darling nosed his small boat into a fresh swirl. 4 The whale that had left
it was visible 40 feet below, suspended head down in pure blueness with its
15-foot-long arms, or flippers, flared out to either side like wings. 5 "That's
the posture humpbacks most often assume when they sing," Darling said. 6 A
hydrophone dangling under the boat picked up the animal's voice and fed it
into a tape recorder…
7 With the notes building into phrases and the phrases into repeated themes,
the song may be the longest – up to 30 minutes – and the most complex in the
animal kingdom. 8 All the humpbacks in a given region sing the same song,
which is constantly evolving.