Yiddish uses the Hebrew alphabet, but it uses it in its own way, according to its own rules. Like Hebrew, Yiddish reads from right to left and doesn't use capital letters. But while Hebrew uses consonants only, consigning its vowels to dots and dashes above and below those consonants, Yiddish assigns certain vowel sounds to certain Hebrew letters. These letters are then incorporated into the spelling of the word.
Another thing: Yiddish spelling is mostly phonetic, it is spelled almost entirely the way Standard Yiddish is pronounced. The basis of Yiddish writing is the Hebrew alphabet. Most words of Hebrew or Aramaic origin retain their original spelling.