The Gemara (ibid. 15b) rules that whether one recites Keri’at Shemawithout hearing the words one recites or reads it without being meticulous with its letters, one has indeed fulfilled his obligation. Nevertheless, one should preferably recite Keri’at Shema properly by hearing the words one utters and pronouncing each letter meticulously.

Reading Without Pronouncing the Words
We must point out though that when our Sages rule that if one reads Keri’at Shema without pronouncing the letters meticulously, one has fulfilled his obligation, this only applies to a lack of meticulousness, i.e. not emphasizing a letter one was supposed to emphasize or not spacing words apart when there is a need to do so (as we shall soon explain); however, if one has actually omitted some letters or words from Keri’at Shema, one has not fulfilled his obligation at all.

Thus, if one reads Keri’at Shema hastily and instead of reciting “Ve’Ahavta Et Hashem Elokecha” one recites “Ve’Avta Et Hashem Elokecha” or anything similar to this, one has not fulfilled his obligation of reciting Keri’at Shema.

Emphasizing the Letter “Yod”
One must clearly emphasize the letter “Yod” of the “Shema Yisrael” so that it is not swallowed and sound like “Shema Israel.” The same applies to the to the “Yod” of the word “Ve’Hayu Ha’Devarim Ha’Eleh.” (Likewise, one must be careful of this during Kaddish as well not to pronounce the first two words as “Itgadal Ve’Itkadash.”)

Places Where Words Must be Spaced Apart
One must pause slightly during Keri’at Shema between the words “Asher Anochi Metzavecha Hayom” and the words “Al Levavecha.” The reason for this is for it not to seem that the word “Hayom” corresponds to “Al Levavecha” which would imply that the words of Torah must be on your heart only today and not tomorrow. (Indeed, the word “Hayom” corresponds to the phrase before it “Asher Anochi Metzavecha.”

One must pause between the words “Asher Nishba” and the name of Hashem following it, for if one reads these words together quickly, the word “Nishba” will lose its “Ayin” sound and it then seems that one is pronouncing the word “Nishba” with a “Heh” which, G-d-forbid, refers to capturing. When pausing between these words, one should pronounce the guttural “Ayin” sound as best as one can.

One must pause slightly between the word “Ve’Chara” and the word “Af” so that the word does not sound like “Ve’Charaf.” (From the fact that the Rishonim quote this example of words one must space apart in Keri’at Shema, it is clear that they pronounced the “Kamatz” sound in accordance with the Sephardic and Middle Eastern custom, for if they pronounced it like the Ashkenazim, there would be no concern that the word would sound like “Ve’Charaf.”)

One must pause before any word whose first letter is the same as the last letter of the word before it, such as the words “Be’Chol Levavecha” (for if one does not space them apart, the “Lamed” sound makes the word sound like “Be’cholevavecha”). The same applies to the words “Be’Chol Levavchem,” “Al Levavchem,” “Va’Avadtem Mehera,” and “Etchem Me’eretz” among others. In many Siddurim published today, there is a noticeable sign in those places where words must be spaced apart.

Similarly, one must pause between a word that ends with a “Mem” and a word that begins with an “Alef,” such as, “Ve’Limadtem Otam,” “Ukshartem Otam,” “Ve’Samtem Et,” “Ur’eetem Oto,” “Uzchartem Et,” and “Va’asitem Otam” so that it does not sound like “Ur’eetemoto” or “Ve’Samtemet.”

“Le’Ma’an Tiskeru”
One must make sure to pronounce the letter “Zayin” well when reciting the word “Le’Ma’an Tizkeru” so that it does not sound like “Tiskeru” (which refers to renting or leasing). The same applies when reciting the word “Uzchartem.” (See Shulchan Aruch, Chapter 61)