How does one determine what da-compound to use if there is no fixed preposition in a sentence? For example: Sie konnte ihre Brille nicht finden, denn sie saß darauf. How can I know what pronominal adverb to use?
Maybe there's no fixed preposition in your sentence, because you don't know it. Some verbs tend to prefer to be in company of certain prepositions (which, again, highly depends on the context). By that I mean, that you even something need to learn the pairs
Don't forget to learn the case the preposition has.