The Shulhan 'Arukh brings down as Halakha that one must be very particular about the rules of the Daghesh, when it emphasizes a letter and when it has to be pronounced more softly. One has to be careful about the Shewa Na' and Nah, and the rules pertaining to Tenu'ah Qetannah and Gedolah, which also affect them.
Regrettably, the majority of people are not careful about it, indeed, are unaware of the rules and regularly pronounce the opposite way. Unfortunately, they often go on to teach children who also learn to read the words incorrectly. It is very important to find someone who is an expert in the rules of reading and to learn from him.
It is not only during the recitation of the Shema' that it is important to read all the words correctly. Even during the rest of the prayer, including the Pesuqei Dezimra, one must strive to read in accordance with all the rules. Hazzanim must be particular to read correctly at all times, especially when reading from the Torah.
(See Shulhan 'Arukh with Rama, 61:23. Kaf Hahayyim ibid., 61)
Shva na nach and nad, shewa na' and Nah, how to read the prayer and shema correctly, halacha