What is the meaning of "Yod'ei Vinah la'Itim"?
'Rashi' (from Shir ha'Shirim Rabah 6:4), Malbim: They know to give counsel according to the matter at hand. 'Rashi' - David needed to take counsel how to strengthen his kingship, due to Beis Sha'ul.
Radak and Malbim, citing Bamidbar Rabah 13:16: They know when to make leap years and to fix the months (when it should be 29 days, and when 30). They know to calculate the Tekufos (seasons) based on the Mazalos. Radak - it is mentioned here, for the king needs to take counsel with them, for he fixes leap years and months, like we find regarding Chizkiyah (he decided to make a leap year on Adar 30, against Chachamim's will ? Pesachim 56a). This is "Lada'as Mah Ya'aseh Yisrael" ? they will know when the Mo'adim occur.
Radak citing R. Yonah: They knew monetary laws. The same applies to "Yod'ei ha'Itim? Lifnei Kol Yod'ei Das va'Din" (Esther 1:13) and "v'Es u'Mishpat Yada Lev Chacham" (Koheles 8:5). The king needed to take counsel with them regarding monetary laws 1 .
Vilna Gaon: They know to contemplate every matter according to the time. E.g. Achitofel never prayed the same Tefilah twice (Yerushalmi Brachos 4:4), for Tefilos change according to the time.
Radak: According to this, "Lada'as Mah Ya'aseh Yisrael" is not connected to "Yod'ei Vinah la'Itim." It should say "to know what the king will do"!
What is the meaning of "Rosheihem"?
Vilna Gaon: They are heads of Sanhedrins.
Malbim (7:12): The total number of Yotzei Tzava in Yisachar (7:2-11) was 205,000 1 . They had 200 leaders of 1,000. They came in the name of the entire Shevet to declare David king.
This is like Malbim's opinion (7:6), that Binyamin [and everyone listed in 7:6-13] is from Bnei Yisachar. (PF)
What is the meaning of "v'Chol Acheihem Al Pihem"?
'Rashi': They followed their (the leaders') counsel.
