What "Tziyunim" should you make?
Rashi #1: Signs in the good ways of your fathers.
Rashi #2: The simple meaning is, make signs 1 for the path you took from Eretz Yisrael to Bavel, so you will return on the same path. I.e. surely you will return from there.
Radak citing Targum Yonasan: Remember the good deeds of your fathers.
Malbim: When we prepare public roads, we make markers at every crossroads saying to where the road leads. When the path is more fixed, they plant Tamrurim (date trees) on both sides, so people know that the path is between the trees. When it is fixed more, it is paved with stones. For Bnei Yisrael who seek the path to Eretz Yisrael, you will find it via the markers, and even more via the date trees, and even more, put your heart to the stones. All these are found on the path you went.
Radak: E.g. rock piles, or a stone marker.
What is "Tamrurim"?
Rashi: They are small date trees planted for a Siman on the road.
Rashi citing Targum Yonasan: Pray with bitterness of soul.
Rashi citing Menachem: It is an expression of Mo'al (raising). He similarly explained "v'Simaros Ashan" (Yo'el 3:3).
Radak: Also this is making signs. The matter is repeated in different words. It is an expression of height, like "va'Yismarmar Elav" (Daniel 8:7). I.e. tall piles 1 .
They should be tall, so they will be intact 70 years later. (PF)
Why does it say "Shisi Libech"?
Radak: If you will not make signs, put to your heart to remember the path you took to exile, for you will return on the same path. All this is a metaphor for consolation 1 .
I.e. very few of those exiled now will be alive to return 70 years later. However, all should realize that the exile is not permanent. (PF)
Why is it written Halachti, and we pronounce it Halacht?
Rashi: Whenever you went, I (Hashem) went with you.
