A. Echad (“one”) is the crucial word in the sentence. The emphasis we place on it shows that the oneness of God (His uniqueness, His constancy, His indivisibility) is the main principle of Jewish faith.
The poetic phrase in Adon Olam says V’hu echad v’ein sheni – “He is one and there is no second (god)”, a Jewish response to the ancient doctrine of dualism. By extension it also refutes the trinitarian idea of three persons in one.
In addition, we could read into the echad an affirmation that only one being can be God and no matter how great a human being is, he cannot stand with God. An egotist tends to put him- or herself on a par with the Almighty – “God, maybe You are God, but I’m God too”. The Sh’ma insists that only HaShem can be God.
That’s why do we extend the dalet when we say echad, because when writing the word it is so easy to make a mistake and write a resh instead of a dalet… and resh makes the word into acher, “someone else”.