Why is Chasidav written without a Yud?
Rashi (from Midrash Shmuel 5:17): It can be read Chasido. The writing and pronunciation teach whether for an individual or a Rabim. The same applies to "Merivav" (verse 10).
Radak: Sometimes the Yud indicating the plural is omitted. The same applies to "Merivav" (verse 10).
What guarding does the verse discuss?
Radak: The Chasidim's legs are not stricken.
Mesilas Yesharim (Perek 20): If one guards properly his thoughts, investigates [his deeds], and has Bitachon, nothing bad will happen to him.
Bi'ur ha'Gra: "He'Charash veha'Masger" (Melachim 2 24:16; 1,000 great Chachamim who were exiled to Bavel 11 years before the Churban), they themselves [survived 81 years in Bavel] and returned to build Bayis Sheni.
Malbim: The world exists only due to Hashem's Hashgachah. It guards the Chasidim; their happiness will not lapse.
R. Yochanan (Yuma 38b): If a person passed most of his years and did not sin, he will not sin after this.
D'Vei R. Shila (Yuma 38b): If an Aveirah was presented to a person twice, and he did not sin, he will not sin after this.
What is the meaning of "Yidamu"?
Radak #1: They are quiet from their excitement and evil.
Radak #2: They are cut off. This is like "Al Tidamu ba'Avonah" (Yirmeyah 51:6). It says "ba'Choshech", like one who walks in the dark and falls.
Aderes Eliyahu: This refers to Galus Madai. Haman went [to ask to hang Mordechai before dawn], in darkness, and became like a deaf-mute (could not refuse) when the king told him to honor Mordechai.
Why does it say "Ki Lo v'Ko'ach Yigbar Ish"?
Radak: If Hashem wants to put them in darkness (affliction), their strength and wealth will not help them.
Bi'ur ha'Gra: This refers to exile among Nochrim. Giborim [fell] b'Yad Chalashim - "v'Orarti Vanayich Tziyon Al Banayich Yavan" (Zecharyah 9:13). Aderes Eliyahu - Ish hints to Rabim b'Yad Me'atim. The verse concludes "v'Samtich k'Cherev Gibor" - all the Yevanim [will be defeated] like one man.
