English: “You shall surely tithe” — this teaches that you tithe one tithe in a single year and you do not tithe two tithes in a single year.
On the daf / in context: In years 1, 2, 4 and 5 of the shemittah cycle one separates ma’aser sheni; in years 3 and 6 one separates ma’aser ani; the two never coincide in the same year.
Type of Learning
Derash on the verse (Sifrei) — only one of the secondary tithes is separated each year.
What Is Learned
In any given year one separates either ma’aser sheni or ma’aser ani, but not both.