When reading the first words of Shema' Yisrael, one must be particular to pronounce the letter Yod of "Yisrael" correctly. Some people have the tendency to incorrectly pronounce the Yod as an Aleph, or to swallow it, and say "Shema' Issrael". Similarly, the Yod in the word "Wahayu" should be emphasized correctly so that it does not sound like "Wehau".
There are some Hebrew speakers who drop the letter Heh and pronounce it like an Aleph. Care must be taken to avoid this and to say it correctly.
This rule applies across the board when reading the Shema'. One must be careful not to swallow any letter, or to pronounce it like another letter. For instance, those who pronounce the letter Beh (Beth) without a Daghesh as a "V", must be particular not to pronounce it in "We-Ahabta" as "Ahafta", and instead of "Nafshekha", "Navshekha". Similarly one must not swallow the Alef in "Me-odekha" say "Modekha", and so on.
(Shulhan 'Arukh 61:18. Mishnah Berurah, ibid., 32)