What is the meaning of "Achalai"?
Rashi: They are my prayers, like "Achalei Adoni Lifnei ha'Navi Asher b'Shomron" (Melachim II, 5:3). The Aleph is part of the root; sometimes it is omitted, like the Aleph in "Achvasi b'Azneichem" (Iyov 13:17), "Asuch Shamen" (Melachim II, 4:2).
What are "Chukim"?
Radak (1): They are Mitzvos whose reason was not revealed, e.g. expelling Temei'im, annulling vows 1 and similar Mitzvos.
Malbim: They are Mitzvos without a reason; they oppose the Yetzer of the heart. The Yetzer contests them, since their reason is not known.
Nedarim 73b gave a reason for annulling vows - a wife always intends that her vow be valid only if her husband will consent to it. Perhaps Radak refers to a father's Hafarah of his daughter's vows. Shalmei Nedarim (66b) says that it is because he has the right to marry her to any man he wants. (If he could not annul her vows, she could forbid herself to certain men, or cause that men will not desire her. - PF)
What is the meaning of "Yikonu Derachai Lishmor Chukecha"?
Radak: Worldly engagements should not distract me from guarding Your Chukim. Guard me from this, so my heart and self will be free to guard them!
Malbim: As long as a man's ways are not complete with Hashem, it is easy to heed his Yetzer ha'Ra and transgress Chukim. My ways should be prepared, in order to guard the Chukim!
