The following is a text translated from "Ritreglur" by Freystein Gunnarsson, from 1929. This is from before the spelling reform that turned Z's into S's in Icelandic, and so here are the rules that used to exist for when to spell with a Z
The Icelandic text was written using old rules for comma usage as well as some old vocabulary and phrasing. As such it was more difficult for me to translate as usually I translate modern text. In the most difficult parts, due to things like conflicting meanings being found when I looked up words (ex. usage in the text/old meaning versus modern meaning) or confusion on my part, and also in some parts due to the reader likely being more familiar with the Icelandic terms than the English, I have left the original Icelandic in. I have also taken various measures to make it easier to read, as the original was basically walls of text. I’m fairly sure there are also typos that I haven’t caught, as for the most part this was translated in one sitting without a beta.
The original book pages will be eventually linked in the future (they are outside of copyright) for future self translation checking purposes and for those who find such things easier to read in Icelandic than in English, but for now there will be none.
Other note: Often in this they are talking about “supine” form, but after talking about it and looking it up it seems it’s actually better to translate it as “past-participle”, and there’s also some conflicting meanings I found along with that (just for example, Wikipedia may say “it’s the same as past-participle” on the Icelandic version, but in English these two terms are actually different?).
xxxAbout z.Part 37It is pronounced like s, and written as it when ds, ðs, or ts happen to stand together in a syllable, but aren't heard unless the s-sound is in clear pronunciation.In the beginning or ending of words z is not written. -- Written here are the main rules, for when one should write z and when not.1. One shall write z in various derived words when the bulk of the stem-word ends on d, ð or t but the derived ending starts on s, and d, ð or t aren't heard in pronunciation, ex. anza (and-sa), hanzki (hand-ski),
(Note: derived words are ex. nouns [beautiful] derived from adjectives [beautifully]. Words where part of the word is from something else originally and may not mean something by itself but was still used to form a new word. Look up elsewhere how Icelandic forms new words for more info.)pg. 49kvikinzkur (kvikind-skur), lenzka, -lenzkur, ex. is-lenzkur (country), unz (und's before und-es), venzl, venzlaður (vandi, compare vandamaður), vinza (vind-sa), vinzli (vind-sli), vonzka (vond-ska); --brigzl, brigzla (brigð-sl), -firzkur, ex. vestfirzkur (firð-skur), gæzka (gæð-ska), herzla (herð-sla), hirzla (hirð-sla), lýzka, mállýzka (lýð-ska), nízka (níð-ska), reizla (reið-sla), tízka (tíð-ska), þýzka, Þýzkaland, þýzkur (þýð-sk, compare Þjóðverji), vazla (vað-sla), verzla, verzlun (verð-sla); --beizla, beizli (beit-sla), brezkur (bret-skur), gizka (get-ska), gæzla (gæt-sla), helvízkur (helvít-skur), józkur (jót-skur), marz (Latin martius), neyzla (neyt-sla), skozkur (skot-skur), spotzkur (spott-skur), veizla (veit-sla), vizka (vit-ska), þrjózka, þrjózkur (þrjót-ska).One shall carefully note, to not write z inside words when ð is audible before sl, ex. eyðsla, beiðsla, hleðsla, leiðsla, meiðsli, íðsla, reiðsla. Pay attention and remember that reizla = vog (weighing, means of weighing, ex. scales) and reiðsla = að reiða einhvern (to give someone a ride).One may not instead write z in the ending of words or before a definite article (the word "the"), when ds, ðs, or ts stand together, even if indistinctly heard in pronunciation, ex. landsins, bandsins, borðsins, bragðs, skortsis, herts (hertur).On the other hand one may write z in derived words with the endings sk or sl, when the stem-word ends in neither d, ð, or t. Ex. dirfska, reykvískur, amerískur, elskur, danskur, enskur, franskur, spanskur, sænskur, pg 50írskur, rómverskur, drægsli, hneyksli, eymsli, reynsla, rennsli, vinnsla, þensla, ærsli.Some write the words dans and desember with a z, but this is not necessary. 2. One shall write z in the superlative forms (superlative is ex. the -est ending and meaning in great-est, versus the "comparative" form of the same word which would be great-er, and the normal form which would be great) of some adjectives and adverbs, when the stem ends in d, ð or t and the resulting superlative ending is stur or st, ex. beztur, bezt (bet-stur), elztur, elzt (eld-stur), helztur, helzt (held-stur), hinztur, hinzt (hind-stur), nyrztur, nyrzt (nyrð-stur), síztur, sízt (síð-stur), stytztur, stytzt (stytt-stur), yztur, yzt (yt-stur). --In stytzur tzt is written, because there is the ð-sound clearly in pronunciation. Furthermore are fæstur, hæstur, næstur, smæstur and so forth, although there is no d, ð, or t in the stem.3. One shall write z in the endings of verbs where that which follows it, like st after d, ð, or t, is not heard in pronunciation and is mainly in middle voice.
This may be done in all words when writing tzt, where tt comes before st ex. hefur hitzt (hitt-st): furthermore ðzt in (sagnbót miðmynd) middle-voice form when the stem ends in ð, but in (sagnbót gjörmynd, modernly germynd) active voice form they end in tt, ex. greiðzt (greiða, greitt), compare section 34. When st or sst are right before st it is best to write the full stems, but these word forms are few, ex. hann hefur festst (festast), þau hafa kysstst (kyssast).
(Clarity note: When the stem of a word ends in ð in the middle voice conjugation of it, but that same stem ends in tt when conjugated in active voice, those rules are in section 34. As for sagnbót supine vs the two miðmynd middle-voice and germynd active voice, I don't understand the differences clearly so the translation of those terms are probably wrong)
Section 34: About t (Note: Only translating the relevant information)
It is soundless in some relationships, illustrated here:
4. Before zt in superlative adjective and verb-forms, but tzt shall be written, when the stem ends on tt and st is added, ex. stytztur (stytt-st), breytzt (breytt-st), and so forth. -- When ð is in infinitive-form verbs, it appears in middle-voice, even though the active voice form ends in tt. One shall write læðzt (læða, lætt), breiðzt (breiða, breitt), veiðzt (veiða, veitt).
— Moreover it is good to remember this, that the helper-words hafa and geta (taka með sér sagbót) appear along with the past-participle, and thus one shall write z in middle voice in the word alongside them (Note: they are not in past-participle form themselves, but the verb after them, ex “I have helped”, is in past-participle[?]). The verbs fá, vera and verða take the past-participle (Note: ex. “fá - to receive/get, vera - to be, verða to become”, I have gotten/been/become. They all appear alongside “hafa - to have” as described above) and they can likewise take the infinitive, but that is easily-recognized by the infinitive marker að, ex. þau fá að talast við, hann er að hamast, hann verður að losast.b) In 2nd person plural in middle-voice, both in the present and past tense, all moods, ex. þið (þér) látizt (látið-st), þið (þér) létuzt (létuð-st), þið (þér) farizt (farið-st),pg 52þið (þér) færuzt or færizt (færuð-st or færið-st), setjizt þið (þér), varizt þið (þér).Here are identical forms in 1st and 3rd person subjunctive, but in them one should not write z, because their active voice forms don't end in ð, ex. ég látist (láti-st), han (hún, það) látist (láti-st), þeir (þær, þau) létust or létist (létu-st or léti-st). And this is easy to notice, if attention is paid to the corresponding person in active voice.c) In the present and past middle voice, when the corresponding forms end in t or d, tur or dur in active voice [when in the middle-voice ending ur falls out and becomes r]. Such forms happen to come in the singular of all persons (note: personal pronoun forms as illustrated below, not sure how to translate), and are in verbs that are always monosyllables, ex. ég læzt, þú læzt, han læzt (læt-st), þú lézt, han lézt (lét-st), ég sezt, þú sezt, han sezt (set-st), ég flyzt, þú flyzt, hann flyzt (flyt-st), ég batzt, þú batzt, han bazt (batt-st, ex. hann batzt í því), ég stenzt, þú stenzt (stend-st), ég vinzt, þú vinzt, han vinzt (vind-st, ex. vindast í lið), ég helzt, þú helzt, han helzt (held-st).Thus shall verb-forms be written, if the stem ends in ð which is not heard in pronunciation, ex. ég bregzt, þú ræðst, hann ræðst.d) In 2nd person singular in the middle-voice past-tense, strong verbs shall be written with z, even though the stem ends neither in d nor t. In these forms they stand so, because of ending in active voice in t in the past (older Icelandic), ex. þú gekkt, þú stakkt, þú bart. This t has since changed into st, so now they arepg 53said and written in active voice þú gekkst, þú stakkst, þú barst. Here they are (vandhæfi - a word no longer used?) difficult to manage in the written form, as active and middle voice are identical in pronunciation, active voice shall be written with s, but middle voice with z, ex. þú gekkst laga leið (from ganga), þú gekkzt upp við lofið (gekkt-st, from gangast), þú stakkst þig á nálinni (from stinga), þú stakkzt á höfuðið (stakkt-st, from stingast), þú barzt þinn kross (from bera), þú barzt mikið á (bart-st, from berast).-- But these middle voice forms are few and seldom-used, and so one must (þess ber að gæta) take care to not write z in these active voice personals (note: again personal-pronoun forms), ex. þú laugst (ljúga), þú skarst (skera), þú laukst (ljúka), þú varst (vera), and so forth.ð) In active voice endings one shall never write z except in 2nd person singular past-tense in monosyllable verbs with t or tt in the stem, and that end in the 1st person in t or tt, ex. ég sat, þú sazt (sat-st), ég hét, þú hézt (hét-st), ég grét, þú grézt (grét-st), ég batt, þú batzt (batt-st), ég hratt, þú hratzt (hratt-st), ég vatt, þú vatzt (vatt-st). The same manner goes with ég veit, þú veizt (veit-st), because such a verb has past-tense forms in the present.