I am a bit stuck on that one.
Some adjectives end with one and the others the other one. How do you know?
Nationalities are -isch, I noticed. In some parts of Germany -ig is pronounced “ich”. It can be even more confusing.
Ich bin Französisch, ja, das ist doch möglich. Findest Du das unglaublich? Es ist aber auch ganz wichtig. 
Experience and unexplainable logic! 
das habe ich nicht so gut verstanden 
not all nationalities are _isch, not by far : Amerikaner, Japaner, Kanadier, Italiener. Spanier, Grieche etc
you mean probably adjectives describing languages/nationalities and their corresponding nouns?
In Middle and Northern Germany most people use that -ich pronounciation (go to HH - i.e. Hamburg and you will hear almost only _ich’s). It is Hochdeutsch pronunciation, however.
in comparison with many other languages, German is crystal clear. Don’t speak with me about Dutch syntax or doubling of consonants in Italian - let’s leave unconsidered the strange agglutination rules in Bahasa Indonesia or something. I might get maaaaad 
So @Hydroptere, in other words, you don’t know neither Yes I meant the words in -isch relating to countries.
That was a random sentence trying to highlight some of those words.
Thanks @Andrea_Mo3, you confirm what I thought.
So, we arrive to this ? : learn it stupid 
Probably there is a theory behind all this, but we are too lazy to look after it or study it, hoping that we’ll figure it out through practice!
This is basically my story with the whole German grammar! 
na dann: “ich bin Franzose”. " ja, das ist doch möglich" huh? -why should be not be possible to be French??? (nein, ich finde es nicht unglaublich). Ich kapiere es noch nicht 
about the most “-lich” adjectives … I’ve remember only that at school they told us, they show a quality (rötlich, fröhlich, tröstlich. untröstlich etcetc). (An adj like rötlich shows in fact a sort of diminishing of the main quality - of being red… hmmm)
You forgot the adjectives ending in -bar. Many -lich adjectives are nowadays replaced quite often by these -bar ones… for ex erklärbar instead of erklärlich…
bah… I’d have to look for school books … but I dont remember the class… 3rd? 4th? 5th?
I don’t know German but I know some related languages. Ig was seemingly “-ic” in Ancient English, in the old days actually being “-like, alike” (from “likur, -ligur”):
demonlike --> demonlik —>
= demonic (modern English)
= demonlig (modern Nordics & German; Old English).
We only have a few of these left in modern English and at least some of them aren’t of English/ancient Germanic origin, ex. “angelic, heroic, horrific, terrific, pathetic”. In English this -lig eventually further evolved into -ly and then -y in a few words, and completely fell off or was replaced by Latin/French in others:
brother-like --> brother-lik, brotherlig —> brotherly.
manlike —> manlig —> manly.
swarthlike --> swarthy (swarth, “svart”, being our original word for black)
water-like —> waterly —> watery —> aquatic (replaced by French/Latin)
time-like --> timey —> temporal (replaced by Latin)
Note that the i “ee” and y are actually the same sounds, y is just shorter than ee. Likewise K and G are the same except for that the vocal chords are used on G and not on K. That’s why they so easily turn into each other, historically, across all languages, and why -lik became ly / lig. Naturally, “ch” is made very close/similarly in the mouth compared to K.
The isch, if it doesn’t come from “ig” in some way, was possibly -skr, and was probably the plain adjective ending (I don’t know as much about the origin of this one) which became -sh or fell off in English:
Enskr, (Engil-ligr/Engil-likr) Engill (Old Norse) -->
Enskur, (Eingilskur), Eingil (Faroese)
English (Angelic), Angel (English)
Engelsk (Änglalik), Ängel (Swedish)
Englisch (Engelhaft?), Engel (German)
In Faroese/Icelandic there are two different words for “English”, one for “Englishman” (eingil) and one for “English language” (ensk); in most of the other Germanics only this “engil” (the one that originally meant “angel”) seems to be left.
Note that even today, “rs”, “k(vowel)” and so on in most of the Germanic-Nordic languages turns into either a sh or ch sound. So the German side of things makes sense to me from what little I know anyway.
On the same pattern as “English” we have Spanish (spansk), French (fransk, franc as in francophone) and so forth; the sh apparently becoming ch in English in some cases… However if we’re talking about, say, “American, Canadian, African, European”, that -an is an ending from French or something as far as I know, so if German, Swedish etc borrowed it or replaced it with the more original -sk ending (amerikansk, kanadensisk, afrikansk, europeisk) is just according to when in time the word was borrowed.
In English we also have “China, Chinese; Faroe, Faroese”, that -ese was actually taken from French/Latin, while Swedish has “Kina, kinesisk (färöisk)” with the same old -sk ending. In some words we even have both: Indonesian (ese-ian) = indonesisk (-sk).
As far as I understand, adjective end with -ich when they are formed from nouns by adding “-lich” annex. It is the most coomon way to get an adjective from a noun.
“-isch” annex is only used when the noun is the name of a territory: a country (amerikanisch), a federal unit (bayerisch), or even all the world beyond the border (auslandisch).
As for “-ig” annex, it is maybe more ancient one, as risgrynsgrot suggests, because words ending with -ig are not formed from any contemporal German nouns. E.g.: there is “wichtig”, but no “Wicht” (there is das Gewicht, though); there is “hungrig” but no “Hungr” (der Hunger); there is “richtig” but no “Richt” (das Gericht, though); there is “fertig” but no “Fert” (and I cannot think of what the related word is).
This may mean that -ig adjectives were formed from some archaic nouns, and in that time -ig was used instead of -lich. What this mean for a learner is that -ig words must be memorized, and -lich words you can freely make from any noun if you like it (but be ready to appear lacherlich).
I don’t think -isch is limited to territory words. There’s also fantastisch, logisch, typisch, and others.
This is even for Germans difficult. It is just something you have to remember where -isch and -ich is used. You have to also pronounce it correctly for the -isch is a harder sound than the -ich. There is a difference in sound between Kirsche (cherry) and Kirche (church) and you can hear it when a German person speaks it.
The -ig ending is, in proper German - or High German, always -ig, and never -ich. The -ich (ig) is spoken in the northern part of Germany but you will never see it written like that anywhere.
I believe that the opposite is true: that word-final -ig in proper High German is always pronounced “-ich”.
This is the pronunciation used in the Duden, for example. See e.g.
http://www.duden.de/rechtschreibung/Koenig
and the pronunciation is written [ˈkøːnɪç] and the little speaker says it like “Könich”.
Other -g’s at the end of words are -ch only in the north (e.g. my native “Fluchzeuch” is definitely non-standard), but for -ig this “ch” pronunciation is standard.
And I agree that “sch” and “ch” have to be kept distinct in the standard language (e.g. Kirsche/Kirche) but that there are many native speakers who do not have the distinction and who have to learn which words take which spelling. (And there are also hypercorrections, where people say e.g. “Fich” because they think it sounds more standard than “Fisch”.)
ahm, not really
“dämlich” but there is no “däm”? “unglaublich” but there is not “glaublich” etc etc etc
“fertig” is a root word (9th century fertīg), it is not derived from something else
(https://de.wikipedia.org/wiki/Etymologisches_Wörterbuch_der_deutschen_Sprache)
Hi sircemloud,
I don’t know if the answer is still relevant after all this time but here you’ll find a fairly detailed (German) explanation: http://www.duden.de/sprachwissen/sprachratgeber/adjektive-auf--i--ig-isch-lich--i-
If you want to practice a few of these suffixes, you can try this page:
http://www.pahaa.de/Links_D/Rechtschreibung/index_rechtschr_ig-lich-isch-8_34_M.htm
Thanks @risgrynsgrot
I had no idea that the Estonian word “deemonlik” (demonic in English) would have been derived from old English.