Some teachers (and their books) are used to emphasize that there are certain structures used in "written German". For instance, I was told that Präteritum is rather used in texts and not so common in verbal comunicacton, and the same case for indirekte Rede.
How different are Spoken and Written German? Should they be learnt bearing these differences in mind?
Does using Präteritum and phrases like "Er hat mir gesagt, er sei nicht geschwommen" by speaking sound strange?
If the answer to the first question is "yes", then: which other characteristics are typically "written German".