Hashiveinu does not feature a ki-clause: its words of bakasha run up all the way to the chasima. If we are right, this would indicate that what is asked for in this beracha is a one-time favor, and that the need for this favor is fleeting. Yet, what the beracha asks for is that we be returned to Torah; that we be drawn closer to His service, meaning that we be made to serve Him better; and that we be reverted to a state of complete repentance before Him. On the face of it, these are not one-time needs, but perpetual ones. Our devotion to Torah needs to be steadfast, and to persist continually and not let up. How, then, can this bakasha rightfully be construed as having a fleeting nature?
But I think that this line of reasoning misses the point that we are asking for complete return to G-d’s service (teshuva sheleima), and that we are asking for assistance in achieving true teshuva. True teshuva is everlasting; it is not provisional or temporary.
To see this, we need to reflect on the core essence of teshuva. We need to be reminded of the Rambam’s assertion that teshuva incorporates regret and consummate abandonment of the transgression. He characterizes this repulsion-ridden distancing of oneself from one’s erstwhile foibles and offenses as carrying with it a high degree of zeal and intensity. So zealous and determined is the repenter, the Rambam says, that the One Who Knows Things Hidden – (kiveyachol) G-d Himself – can bear testimony on this individual that he will never repeat his offenses, going forward. We may be weak and never entirely sure that we will not succumb to temptation in the future and revisit the flaw – the fault – for which we are now repenting. Nevertheless, what we seek and try to achieve when we engage in teshuva is thorough disassociation from the forbidden act that we performed (or failed to perform, as the case may be). We want our detachment from it to be so consummate, and to embody such finality, that it will be (all but) certain that we will never return to our wanton ways – at least as far as this particular shortcoming/transgression is concerned.
So from the point of view of our thoughts and our intentions, those that are uppermost in our mind as we recite the petition for teshuva (in hashiveinu) – from this vantage point, we are asking for (what we hope will be) a one-time favor. We are asking to be returned to Him so consummately that the need won’t rearise for us to ask for this in the future. It is a fleeting request.
Selach lanu, on the other hand, asks for forgiveness. It is predicated on the realism that people commit transgressions for which they require and seek pardon. Consonantly with the prerogative of bechira, it does not importune for the total eradication of sin. This prayer fully expects people to return to itself over and over. It is a perennial request, asking for something that is repeatedly needed. It thus features a ki-clause.