Ist das Jiddische ein deutscher Dialekt oder eine selbständige Sprache?
Wie es der Zufall will, habe ich kürzlich eine ähnliche Frage in einem englischsprachigen Quora-Forum beantwortet. In der Annahme, dass Sie Englisch verstehen, füge ich meinen Kommentar im Original bei:
Is Yiddish basically modified German?
https://www.quora.com/Is-Yiddish-basically-modified-German
It would be more correct to say that Yiddish (which should rather be spelled Yidish) and German are closely related sister languages, almost twins. Both languages began to split in the late Middle Ages, but remained in contact until the 20th century.
Therefore Yiddish and German followed a similar evolutionary path even after becoming separate languages (e.g. middle high German long î and û which were common in both languages at the moment of the splitting, were not conserved in either of them, but but became diphthongs in both).
If we compare with some middle and upper German dialects, similarities are even greater than with standard German. On the other side, Yiddish also has dialects, some of which are closer to German than others. Lithuanian Yiddish, traditionally considered as the most prestigious form, conserves German u, where Polish / Chasidic Yiddish changes it to i (which means that e.g. in = in and un = and are both pronounced "in"). The change does not appear in writing, but makes spoken "pojlish" more difficult to understand for speakers of German.
Of course there are significant differences between Yiddish and German. To name just a few:
Yiddish (at least most of its dialects) has lost vowel quantity as a phonemic feature, while it is conserved in German.
Yiddish vowels and diphthongs often diverge form their German equivalents (e.g. y. y. hobn vs. g. haben, y. oyf vs. g. auf).
In some cases, Yiddish has diphthongs where German has long vowels (e.g. y. Poyln vs. g. Polen).
Unlike German, Yiddish does not “harden” voiced consonants in final position.
Yiddish has syllabic consonants, unknown in German (except southern German dialects).
Yiddish has radically “normalized” German Umlaut and Ablaut, both in nominal and verbal morphology. In general, the system is simplified (e.g. y. ich es, du est vs. g. ich esse, du isst), but in some cases Umlaut has been extended to cases where it is not present in German (e.g. y. kats, plural kets vs. g. Katze, plural Katzen).
Though conserving all German pattern for the formation of plurals (-en, -er, -s, Umlaut etc.), Yiddish has changed plurals of many, if not most nouns (e.g. y. gartner vs. g. Gärten).
Yiddish has “regularized” the position of the verb, putting it usually directly after the subject (e.g. y. ich bin geforn mit mayn tate qeyn Erets-Yisroel vs. g. ich bin mit meinem Vater nach Israel gefahren).
Yiddish (as upper German dialects) has abandoned the synthetic past tense (e.g. y. er hot gezogt vs. g. er sagte), and changed the pattern of future tense (e.g. y. ich vel zogn vs. g. ich werde sagen).
Yiddish has lost the double (strong/weak) adjectival flexion (e.g. y. a groyser man / der groyser man vs. g. ein grosser Mann / der grosse Mann).
Yiddish has simplified the case system, but conserves the original accusative ending -n, gnerally lost in German (e.g. y. ich hob lib mayn taten vs. g. ich liebe meinen Vater).
Yiddish conserves many lexical archaisms (e.g. y. mume vs. g. Tante, y. eydam + shnur vs. g. Schwiegersohn + Schwiegertochter), has changed the meaning of some roots (e.g. y. trachtn vs. g. denken), or the grammatical gender (e.g. y. der shif vs. g. das Schiff), and created new words from Germanic roots (e.g. y. flaterl vs. g. Schmetterling, y. shprits vs. g. Dusche).
Yiddish makes considerably more use of diminutives (e.g. jinglekh vs. g. Jungen).
And of course Yiddish has incorporated hundreds of Hebrew (and some Aramaic and Slavic) lexemes unknown in German.
I do not mention purely (ortho-) graphic divergences, such as the use of different alphabets. Script is not a part of language, but rather a secondary system. Writing Yiddish in Latin characters would not make it "more German".
In recent times, Yiddish spoken by the Ashkenazi community in the US has undergone strong English influence, both lexically (e.g. der botl < the bottle; tsi votshn < to watch) and phraseologically. Spoken New York Yiddish often sounds like a calque or a literal translation from English, and consequently less German than “classical” Yiddish.
The result is that at present Yiddish and German are not mutually intelligible - at least when it comes to complicated subjects. Of course you can find lots of sentences that sound (almost) identical (the same could be done, e.g. between Italian and Spanish, or Croatian and Czech), but I would challenge any German native speaker to correctly translate a page of one of the great Yiddish novels without spectacular errors.