The 10 most important new spelling rules of the German language:

(Note: examples are given in green)

    ss – ß: ß is used only after a long vowel or diphthong.: Maß, schließen, reißen, draußen ... After a short vowel always use ss instead of ß: messen, Schloss, gerissen, Riss, küsst ...
    Letters are no longer omitted in compound words: Kaffeeernte, Rohheit, Schifffahrt, schnelllebig ... (exceptions: dennoch, Drittel, Hoheit, Mittag)
    Stamm- and Parallelschreibung: Bändel, Gräuel, schnäuzen, Stängel, Stuckateur, Ass, nummerieren, Platzierung, Tipp ... Plural after -ys: Babys ..., also rau and Känguru.
    Optional assimilation of a few foreign words Delfin, words with phon, phot, graph: Mikrofon, Geografie, words with th: only Panter and Tunfisch, also Katarr, Differenzial, Jogurt, Spagetti, Schikoree, Portmonee ...
    When are words written separately and when are they written in one word?
        verb + verb can always be written separately: lieben_lernen, spazieren_gehen, gefangen_gehalten, getrennt_lebend … (if a new meaning evolves, words can also be written in one word: sitzenbleiben, kennenlernen …)
        noun + verb: Acht geben, Rad fahren ... (not with "faded" nouns like preisgeben, eislaufen, stattfinden, teilnehmen ...)
        adverb + verb: always with ‘sein’ and written separately: da sein, zusammen sein ..., also: allein erziehen, zustande bringen ... (when combined with a participle it can be written either way: allgemein bildend or allgemeinbildend).
        adverb + adjective/adverb: allgemein_verständlich, wie viel, zu viel, so lange … (except of conjunctions: Solange du fern bist, …).
        always written as compounds: irgendetwas, irgendjemand, umso (desto), zurzeit (derzeit)
        adjective + verb: separate if literal meaning (klein schreiben) or combined if new meaning is carried over (kleinschreiben)
    Use of hyphens with numbers: 8-mal, 20-jährig ... (but not with suffixes "~er", "~ig", "~tel": 30er Jahre, 100%ig, 12tel ...) but also 3-fach.
    Use of small letters
        the address in 2nd person can be written in capital letters - but only in letters: du, ihr, dein, euer ... or. Du, Ihr ….
        firm connection between adjective and noun: das autogene Training, das neue Jahr ... (unless with proper names: die Dritte Welt ...); However, you may use capital letters if there is a new idiomatic meaning: das Schwarze Brett, der Blaue Brief … and in technical language: der Goldene Schnitt, Erste Hilfe …
        adjectives formed by proper names before (i)sch: die grimmschen Märchen ... (capitalization with an apostrophe only to emphasize the name: die Grimm'schen Märchen ...)
    Capitalization
        names of daytimes after 'gestern', 'heute', 'morgen': gestern Abend ...
        nouns when words are separated: in Bezug auf, Schuld haben ... (except recht/schuld sein ...). With Recht/recht and Unrecht/unrecht both are acceptable in conjunction with verbs.
        using an adjective as a noun, especially after articles: der Einzelne, als Erster, das Gleiche, des Weiteren, im Allgemeinen ...
        after prepositions (without an article) either is possible: seit k/Kurzem, ohne w/Weiteres ...
        nouns in the infinitive are capitalized after prepositions: auf Deutsch, in Schwarz, für Groß und Klein ...
        word pairs are capitalized too: Jung und Alt, Groß und Klein ...
    Use of commas:

        A comma DOESN'T HAVE TO be used:

        with the conjunctions 'und' or 'oder': Er studiert noch und sie ist arbeitslos. ...
        if infinitives are used in combination with a participle: Zu Hause angekommen legte er sich hin. ...

        A comma MUST be used

        if there is a dependence on a noun: Er hat den Wunsch, Arzt zu werden. …
        with an infinitive with "zu": um, ohne, statt, anstatt, außer, als zu
    Separation of words after phonetic syllables:

        s-t: Fens-ter
        ck like ch: Bä-cker (instead of Bäk-ker)
        single vowels only in medial position of a word: Ru-i-ne, not at the beginning: ü-ber and at the end: Treu-e
        not in compound words: Sonn-abend, Bio-top hi-naus, wa-rum, Pä-dagogik is also possible ...