One must have upmost kavanah while reciting the first verse of the Shema, in accepting the yoke of Heaven, as it is written (Deuteronomy 6:6), “Put these words… in your heart.” Therefore, a person must concentrate in his heart on the words he is saying while reciting the first verse. If he did not concentrate on the words he recited, he did not fulfill his obligation (Berachot 13b; Shulchan Aruch, 60:5, 63:4).
Even a person who concentrates on the full meaning of every word with kavanah must try not to let his mind wander to other thoughts in the middle of the verse. However, b’dieved, as long as he was also thinking of the meaning of the words, he fulfilled his obligation.
Thus, when saying “Shema Yisrael,” it is proper to reflect upon how the mitzvah of accepting the yoke of Heaven is destined for Israel, a nation that was created in order to reveal the belief in the unity of Hashem in the world. The word “Hashem,” which is not read as it is written, is spelled, “yud” “heh” “vav” “heh” and pronounced “A-donai.” While saying it, one should have in mind the way it is pronounced, that Hashem is the Master (Adon) of everything, and concentrate on the way it is written, that He was, is, and will be (Hayah, Hoveh, V’yihiyeh). In reciting the word “Elokeinu,” one should know that God is firm and omnipotent, Master of all existing powers, and that He rules over us (Shulchan Aruch 5:1). When a person says “Echad,” it should be with the kavanah that Hashem is the only ruler of the world, in the heavens and the earth, and the four directions of the world. This meaning is implied in the letters of the word “Echad”: “Alef” – that He is One, “Chet” – signifying the seven heavens in addition to the earth, and “Dalet” – representing the four directions. One must extend the pronunciation of the letter dalet as long as it takes for him to think of the fact that HaKadosh Baruch Hu is One in His world and that He rules in the four directions of the world (Shulchan Aruch 61:6; and see Mishnah Berurah 18).
B’dieved, even if a person did not have in mind the exact meaning of the Name of Hashem and each and every word, but he understood their overall significance – the acceptance of the yoke of Heaven – he fulfilled his obligation.
However, if his mind wandered and he did not concentrate on even the general meaning of the words – the acceptance of the yoke of Heaven – then he did not fulfill his obligation, and he must repeat the words and recite them with their meaning in mind. If he remembers this immediately upon finishing the first verse, he must wait a bit, so as not to be seen as one who is reciting Shema twice, and then repeat the first verse quietly. If he remembers in the middle of the first paragraph, he must stop, start from the beginning of the first paragraph, and recite the whole passage in order. If he remembers in the middle of the second paragraph, he finishes that paragraph, after that goes back and repeats the entire first paragraph, and then skips to the third paragraph, Vayomer. He need not repeat the second paragraph because he already had the appropriate kavanah while saying it and b’dieved, the order of the paragraphs does not prevent him from fulfilling his obligation (Mishnah Berurah 63:14; Kaf HaChaim 17-18).
In order to arouse kavanah, it is customary to read the first verse out loud and to cover one’s eyes with his right hand so as not to look at anything else that might interfere with his concentration (Shulchan Aruch 61:4-5; Mishnah Berurah 17).
. See Bei’ur Halachah 101:1 s.v. “Hamitpalel,” based on the Rashba brought by the Beit Yosef 63:4. The Rashba indicates that one must not let his mind wander and thereby not have kavanah to accept upon himself the yoke of Heaven. However, b’dieved, if he had kavanah and also dreamed in the middle, his dreaming did not nullify his kavanah.