1)    1) The obligation to concentrate on the meaning of the words applies primarily to the first verse of the Shema, so that he will accept upon himself the yoke of the kingdom of heaven. It follows that if he recited this verse without the relevant concentration that he has not fulfilled his duty, and must recite it a second time with the correct concentration, Similarly, if he did not concentrate when saying the verse “Baruch sheim”, he should recite it again with the correct concentration. (Shulchan Aruch and Mishne Berurah 63)

2) If after having read the entire paragraph of “Vehayah im shamoa” until “Vayomer” he remembered that he had not concentrated in the first verse, he should recite it again together with the entire first paragraph and he does not need to repeat the paragraph of “Vehayah im shamo’a”. Instead, he may immediately start “Vayomer”. (ibid)