The choice between preterite and imperfect depends both on factors which are constant for all speakers of Spanish and on the speaker's message and intention in a given context.

Communication is rarely limited to single sentences, so many of the textbook examples could conceivably have the other tense in another context. In all cases, however, the obligatory choice between tenses adds a special nuance to the idea expressed by the verb.

Here are some of the principles governing this choice:

Preterit: Foreground / Focus event, often one which presents an independent event or advances the story line.
Imperfect: Background process on which a (completed) foreground event is "superimposed."

Preterit: Achievement-start or completion is relevant; often momentary events. "Closed box" or "snapshot"--no subdivisions of the event are examined.
Imperfect: Duration, non-achievement--event viewed as ongoing, incomplete, or even interrupted ("was/were verb-ing"). "Open box" or "movie"--the event is subject to subdivision and examination.

Preterit: Sequential development.
Imperfect: Simultaneous development.

Preterit: Repeated event with clear start and finish and a specific number of repetitions.
Imperfect: Habitual event ("used to", "would").

Preterit: Specific time references: ayer, el domingo, el año pasado
Imperfect: General time references: siempre, con frecuencia

Ayer llovió todo el día. Por eso me mojé. [two foreground events]

Ya llovía [background] cuando me levanté.

En la secundaria tocaba [habitual] la guitarra casi todos los días.

Toqué tres veces a la puerta, pero no vino nadie. [preterit for a repeated action viewed as a single event with definite completion]

Anoche cené a las seis. Luego fui a la biblioteca donde me quedé tres horas estudiando. Después volví a la residencia y me acosté a las diez y media. [preterit for series of events, "sequential development"]

Mientras yo te esperaba, tú estabas en tu cama durmiendo. [imperfect for ongoing simultaneous events]

Anoche supo ("he found out") que hoy no tenemos examen. [special translation of saber in preterit--start to know" = "find out"]

Ya conocía ("I knew him"--imperfect for background) a Rodrigo cuando conocí ("I met"--preterit for focus event, "became acquainted with" = "met") a su hermana.