(ג) ושב ה' אלהיך את שבותך. היה לו לכתוב והשיב את שבותך, רבותינו למדו מכאן שהשכינה כביכול שרויה עם ישראל בצרת גלותם, וכשנגאלין הכתיב גאלה לעצמו, שהוא ישוב עמהם...
"And Hashem your G-d will go back your captives" (3) It should have written "and He (G-d) will bring back your captives" (instead of שב - go back, it should have said השיב - bring back.) Our Rabbis learn from here that it is as if Hashem's presence is with Israel (the Jews) in the time of the hardship of their exile. And when (the Jews) are redeemed, (it is as if Hashem had) redemption written about himself, that He would return with them....
**This is a great Rashi, because his question is a question about פשט and why one word is written weird. However, from this question he brings a beautiful message about teshuva -- that when we are suffering in גלות, Hashem is with us. When Hashem returns us to Israel, it is as if He Himself is returning as well, since He has been with us all along.
(ו) ומל ה' אלהיך את לבבך זהו שאמרו (שבת קד) "הבא לטהר מסייעין אותו" מבטיחך שתשוב אליו בכל לבבך והוא יעזור אותך...
"And Hashem your G-d will circumcise (do surgery on) your heart (so that you will love Him)... This is what the Rabbis meant when they said (in Gemara Shabbat 104): "Someone who comes to be purified is helped." Hashem is promising you that if you return to Him with all your heart, He will help you.
**Here Ramban is also coming with a פשט question. In the passuk, it sounds like Hashem is taking away our free choice, because He is going to "do surgery" on our hearts so that we only want to love Him all the time and never want to do wrong! How can Hashem force us to love Him like that? Ramban explains that Hashem is not forcing us. Rather, when a person wants to change and grow closer to Hashem, and he begins the process, Hashem helps him along. Sometimes, a person may want to do teshuva but could be discouraged because they have come so far from Hashem and feel they can never improve! But if Hashem sees the effort, He will help the person by slowly taking away the desire to do bad the more He sees the person is really trying. This is how it is possible for a person to turn his whole life around and really do teshuva.