Lack of Nekudot
Hebrew can be written with or without nekudot (vowel sounds under the letters). When reading without nekudot, however, the reader is forced to infer which nekudot would probably be beneath the letters. In Yiddish, however, the letters themselves usually contain the vowels. For example, rather than the use of the segol nekudah, the letter ע signifies the short “e" sound (as in "פעטער" – feter, or father). Although the kamatz (“uh") and the patach (“ah") remain, they are only used under the letter א when necessary (as in "מאמע" – mameh, or mother). This is also made possible by the additional vowel blends in Yiddish.
Additional Vowel Blends
The Yiddish language includes several vowel blends that do not appear in Hebrew. These include the “double yud" that makes the long “a" sound (as in "אוי וויי" – oy vey, or woe is me), the “double yud with a patach" that makes the long “i" sound (as in "לייכטר" – liechter, or candelabra), and the “vav-yud" that makes the “oy" sound (as in "אוי" – or oy). Each of these sounds occurs often in the Yiddish language, and they help compensate for the lack of nekudot.
Additional Consonant Blends
There are four main additional consonant blends in the Yiddish language that are not found in the Hebrew language: dalet-shin (דש), zayin-shin (זש), dalet-zayin-shin (דזש), and tet-shin (טש). These blends are pronounced, respectively, “dz," “zh," “j," and “tch." In addition, because the letter “vav" (ו) is so often used as part of a Yiddish vowel blend, the double-vav (וו) is used to make the typical “v" sound (as in "קוועל" – kvell, or take joy in).
Additional Pronunciation Rules
Unlike Hebrew, the ב does not have a mapik (dot) in it when it says the “b" sound. Instead, the ב has a line on top of it when it says the “v" sound. This line also appears on top of the פ when it says the “f" sound, rather than the “p" sound. In other words, the weaker of the two letters uses the upper line, rather than the stronger of the two letters using the mapik.