Why does the Torah see fit to insert "Sheish Shanim Tizra Sadecha ... "?
Ramban #1 and Moshav Zekenim #1: It is the way the Torah normally speaks. 1
Ramban #2 and Moshav Zekenim #2 (citing Yerushalmi in Kil'ayim): It comes to teach us that someone who works his land in the Sh'mitah transgresses an Asei 2 (as well as the La'av in the following Pasuk.
Ramban #3 (citing the Mechilta in Mishpatim) and Seforno: When Yisrael do the will of Hashem they will experience one Sh'mitah (per seven years), as the Torah writes "Sheish Shanim Tizra es Artzecha ... "; but if they don't, they will have to keep four Sh'mitos - by leaving the land fallow in the first, third and fifth years, 3 followed by the Sh'mitah in the seventh.
Moshav Zekenim: It is written after "Shabbos la'Hashem" to teach us that if we observe Sh'mitah properly, we will sow our fields; if not, we will have to sell them, and subsequently, also our houses and our daughters (Kidushin 20a).
Oznayim la'Torah: To highlight the miracle that, when one keeps the Sh'mitah, one will be able to to till the land for six consecutive years
Why does the Torah repeat the words "Sheish Shanim"?
Oznayim la'Torah: To teach us that the time that one is prohibited from plowing a cornfield (until Pesach) differs from the time that one may not plow a foeld of trees (until Shavu'os). 1
See Oznayim la'Torah, DH 'Sheish Shanim' #1.
What are the connotations of "ve'Asafta es Tevu'asah"?
Rashbam: It means that, during the first six years of the Sh'mitah cycle, one is permitted to gather the produce of one's own field. 1
Yerushalmi, Ma'asros, 5:2: Based on its juxtaposition ro "u'va'Shanah ha'Shevi'is", it permits harvesting produce in the Sh'mitah that attained one third of its growth in the sixth year. 2