Why does the Torah add the word "ha'Eved"?
Rashi (in Devarim 15:17): To teach that he is able to extend his period of service beyond six years, but an Amah ha'Ivriyah is not.
Kidushin 22a: He must say so while he is still an Eved, at the beginning of the six years (any time before they have terminated). 1
Why does the Torah refer to him here as "ha'Eved"?
Oznayim la'Torah: Bearing in mind that the Torah already referred to him as "Eved Ivri", 1 it refers to him here derogatively (not just as 'Eved', but) as 'ha'Eved', because he chose to extend his years of Avdus
To whom does "Ishti" refer?
Rashi: To his Shifchah Cana'anis 'wife'. 1
Moshav Zekenim (Al Derech Derush): Refer to 21:3:1:3.
See Sifsei Chachmim.
Why does the Torah say "Amor Yomar"?
Kidushin 22a: To teach us that he must say so twice 1
Oznayim la'Torah: The reason for his is because if he were to say so only at the beginning, we would suspect that he does not really mean it and is merely flattering his master or merely pulling his leg; whereas if he would say it only at the end, we would suspect that his statement is based on panic at the sudden realization that from now on he will be forced to bear the yoke of Parnasah on his shoulders (after six years of being sustained by his master).
See Rashi on the Gemara and Torah Temimah, note 43.
QUESTIONS ON RASHI
Rashi writes that "ha'Eved" teaches that an Amah Ivriyah cannot be Nirtza. According to the opinion that "Azno" teaches only a sLa'ave through Beis Din sold is Nirtza, why do we need "ha'Eved"? One is sold "bi'Geneivaso", but not bi'Gneivasah (for her theft)!