1) The main reason for the institution of the psalm “A praise to Dovid” (otherwise known as “ashrei”) in the verses of praise is because it contains the verse “You open Your hand and give each living creature its will!”, in which we mention the praise of Hashem who watches over all of His creations and provides for them. (Shulchan Aruch and Mishne Berurah 51)
2) Since that is the case. If he did not concentrate when saying that verse, he must return and repeat it. If he has already finished the chapter, even if he has already said other chapters of praise, he should repeat from that verse until the end of the chapter. If he has no time to return, he should wait until after the end of the prayers and recite from that verse until the end of the chapter. (ibid)
