The Laws of Prayer /The Blessings of the Shema (67)

1. One must be very careful when reciting the Shema not to put a Dagesh in the letters which are not supposed to have one and vice versa. One must also be very careful which Sheva to pronounce and which not to. (Shulchan Aruch and Mishne Berurah 61)

2. When there is a word which commences with the letter that the previous word finished with (e.g. al levavecha, eschem mei’eretz etc.), one must be careful to pause between the two letters, so that they do not sound like one. The same applies when one word ends with the letter “mem” and the following word starts with an “alef” (e.g. uelimadtem osam etc.). (ibid)