Rashi on Deuteronomy 15:11/רש"י על דברים ט״ו:י״א

1על כן means מפני כן BECAUSE OF THIS.

2לאמר suggests: I give you good advice for your own benefit (Sifrei Devarim 118:2).

3לאחיך לעניך [THOU SHALT SURELY OPEN THINE HAND] UNTO THY BROTHER, TO THY POOR — open thine hand to thy brother, to which brother? To the poor one.

4לעניך written with one Jod denotes one poor man, but ענייך with two Jods denotes two (or many) poor, i.e. it is a plural form.