18. [On Purim], a congregation that does not have a reader who is able to
recite the Megillah with the correct tune, may have someone read the
Megillah without the tune, as long as he reads the words correctly, such
that the meaning is not altered. If, for example, instead of the
words "And Mordechai WAS SITTING" ("Yo'sheiv"), he were to read "[And
Mordechai] SAT" ("Yo'shav"), even post facto ("Bedi'avad") the mitzvah has
not been fulfilled (1).
[In a situation where the reader doesn't know how to pronounce the words
without the vowels, and doesn't know the correct tune], one may write the
vowels and cantillation marks in the Megillah scroll so that he can read
correctly, since it is an urgent situation ("Sha'as Ha'dechak"); [inserting
the vowels and cantillation marks onto a kosher Megillah] is better than
having someone first read each sentence in an undertone from a [non-kosher
Megillah, such as a] Chumash [so that the reader will know how to read the
kosher scroll correctly]. This is because even though he is reading in an
undertone, the person [assisting the reader by] reading from the Chumash
will not be able to concentrate ("Le'kaven Da'atoh") in a way that allows
him to hear [all the words] from the one reading [from the kosher scroll].
Consequently, he will have read only from the Chumash, and will not have
fulfilled his obligation (2). If this did happen, [the one who assisted the
reader] will have to hear the Megillah again from a kosher scroll.
(1) In such a case, the Megillah must be re-read from the place the mistake
was made, but a new blessing is not said. This rule also applies if an
entire word was omitted (Biur Halachah 690:14).
(2) In order to fulfill the obligation, one must read the Megillah, or hear
it being read, from a kosher scroll.