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Distinctions

The essay starts from Moshe's defense against Korach's accusation of monetary impropriety: "I did not take a donkey of echad of them, and I did not do bad to achad of them" (Bamidbar 16:15), one pasuk with two words for "one."

echad (אחד)

634 occurrences. Plainly the numeral one, and beyond that the joining of parts into a single whole. Chazal read a connotation of importance into it: Megillah 28a derives from "the one sheep shall you offer in the morning" (Bamidbar 28:4) that the Korban Tamid must come from the choicest animal, echad marking the pick of the flock. So too "and one of the lads answered" (Shmuel I 17:18), which Rashi, citing Sanhedrin 93b, reads as the outstanding member of Shaul's entourage, namely Doeg the Edomite. The word is bound up with Hashem Himself: Hashem Elokeinu Hashem Echad (Devarim 6:4). On that strength the Midrash (Bamidbar Rabbah 18:10; Tanchuma Korach 7) reads Moshe's "I did not take a donkey of echad of them" (Bamidbar 16:15) as hekdesh, consecrated property, which belongs to Hashem, the Ultimate One: Moshe pleaded that he had not so much as taken a donkey from hekdesh, unlike the standard practice of political leaders, and was certainly not guilty of Korach's charge.

the dispute the two forms answer

Rendering that first clause as "a donkey of one of them," with echad meaning one of the people, follows Rashi and Targum Onkelos. Rashi's own grandson Rashbam, and Chizkuni and Ramban, render it otherwise: "I did not take one donkey from them," echad attaching to the donkey and not to the people, since otherwise the Torah should have written achad as it does in the second clause. It is against that objection that Rabbi Wolf Heidenheim defends Rashi, and his two forms of achad are the defense. (His explanation is quoted by Rabbi Yaakov Tzvi Mecklenburg, HaKtav VaHaKabbalah to Bamidbar 16:15; see also Rabbi Shlomo Zalman Hanau, Shaarei Tefillah 11.)

achad (אחד, construct)

65 occurrences in all. In the construct form it is simply "one of," and by Heidenheim's account carries the same freight of importance as echad: "on achad of the mountains that I will tell you" (Bereishis 22:2), which is Har HaMoriah; "achad of the nation nearly lay with your wife" (Bereishis 26:10), which Rashi reads as Avimelech the king himself; "like achad of the tribes of Israel" said to Dan (Bereishis 49:16), which Rashi and Bereishis Rabbah 98:13 read as one of the distinguished tribes, Yehudah.

achad (אחד, absolute)

Heidenheim's second form looks like a construct but is not; the tell is a following mem ("one from them"), since a true construct swallows the "of." Here the connotation inverts: not important, any ordinary member of the set. "Take with you achad of the lads" (Shmuel I 9:3), a servant of no standing; "he shall be brought to Aharon the Kohen or to achad of his sons the Kohanim" (Vayikra 13:2), that is, any ordinary Kohen as against the Kohen Gadol. This is why Bamidbar 16:15 says echad in its first clause and achad in its second: had the first clause read achad, the Midrash's hekdesh reading, which turns on greater honour, would collapse. Rabbi Moshe Sofer applies the same rule to "behold the man has become like achad of them" (Bereishis 3:22): absolute form, so Adam after the sin stood level with the lowest creatures rather than above them.

Malbim seems to contradict himself here. In Ayelet HaShachar (introduction to Vayikra 69) and to Parashas Tazria 43 he follows Heidenheim's two forms, and his comment to Bereishis 3:22 runs very close to Rabbi Moshe Sofer's, who explicitly follows Heidenheim. But in Artzos HaChaim (Orach Chaim 6:1) he explains the achad of Asher Yatzar, "if one of them were to open or one of them to close," as the "most important" organs, citing Bereishis 22:2 and 26:10 as precedent, though that achad is not in the construct; on Heidenheim's rule it should mean the opposite, which implies that Malbim did not accept Heidenheim's explanation.

ach (אח)

Normally "brother" (also an exclamation of grief, and an "oven"). Rabbi Yonah Ibn Janach and Radak (to Yechezkel 18:10 and in Sefer HaShorashim) insist that in that one pasuk it means "one." Targum and Rashi there keep "brother." Ibn Janach finds a second case in Tehillim 49:8; Radak rejects it and reads "brother."

chad (חד) and chadah (חדה)

Radak (to Yechezkel 33:30 and Sefer HaShorashim): chad is echad with the initial aleph clipped for brevity. That is its only Hebrew occurrence; the other five are the Aramaic of Daniel (2:31, 3:19, 6:3, 7:5, 7:16). Feminine chadah appears nine times, all in the Aramaic of Daniel and Ezra; chada with the definite aleph is not in Tanach but is common in the Bavli and Targumim.

achat / echat (אחת)

The feminine. Ibn Janach (Sefer HaShorashim): feminine of echad/achad, where the expected echadet drops its dalet to avoid dalet and tav back to back. Radak in Sefer HaShorashim disagrees, making achat/echat the feminine of ach and holding that echad/achad have no true feminine; in his Michlol he treats all of them as one stock, ach being echad minus the dalet and chad being echad minus the aleph. Practically, achat (over 200 times) is the standard form and echat (under 70) the positional variant, typically at the end of a pasuk or under an esnachta, so the pair is one word and not two senses.

the biliteral reading

Rabbi Shlomo Pappenheim traces echad, achad and chad to the two-letter root ח-ד, "singularity," what has no counterpart to make it two. From it he derives yachad ("together," many sub-units become one), chad/chidud ("sharp," the whole force brought onto one point), and chidah ("riddle," which takes all of one's mental force sharpened onto a single question). Pappenheim does not treat ach here, holding it never means "one." Menachem Ibn Saruk and Pappenheim keep ach ("brother," א-ח) separate from the one words, which they trace to (א-)ח-ד; Rabbi Shmuel Yehudah Steiger (Avnei Shayish) joins them, all of them naming a closeness and joining together, like brotherhood.

the aleph of echad

Rabbi Yehonosson Eybeschutz writes that Adam spoke Hebrew before eating from the Tree of Knowledge and Aramaic afterwards, and accounts for the switch through this very word. Aleph is the first letter of the alef-beis and has a gematria of one, so it stands for oneness and uniqueness; Hebrew's word for one, echad, opens with it, while the Aramaic chad does not. When Adam defied Hashem's order he slighted Hashem's all-encompassing dominion, an affront to His Oneness, and was demoted to the language whose word for "one" does not carry the aleph.

Rendering guidance

• Default: echad, achad, achat, echat, chad all render "one." The differences are connotative, not lexical, so do not manufacture a distinction in English where the source is not making one.
• Where a derashah is running, mark it: echad as "the one," "that one," or "the choicest"; construct achad as "one of the great," "one of the distinguished"; absolute achad as "any one of them," "a mere one of them."
• Trap: English "one of" absorbs both forms of achad and erases Heidenheim's whole point. When both forms sit in one pasuk, as at Bamidbar 16:15, translate them differently or the verse reads as pointless repetition. On the reading Heidenheim defends: "I did not take a donkey of one of them, and I did not wrong any one of them." Rashbam, Chizkuni and Ramban read the first clause as "I did not take one donkey from them," so a translation there is choosing a side and should say which.
• Chad in a Hebrew context is a clipped echad; in Daniel, Ezra, the Bavli and Targum it is simply the Aramaic for one and needs no comment.
• Ach at Yechezkel 18:10 is a live dispute. Give "brother" (Targum, Rashi) unless following Radak and Ibn Janach, and footnote the other reading.

Sources

Rabbi Reuven Chaim Klein, "Korach: The One and Only," What's in a Word?, Ohr Somayach, 28 Sivan 5783 (2023-06-17): raw text in sources/whats-in-a-word/raw/10975-korach-the-one-and-only.md. Primary loci: Bamidbar 16:15 and 28:4; Devarim 6:4; Bereishis 3:22, 22:2, 26:10, 49:16; Vayikra 13:2; Shmuel I 9:3 and 17:18; Yechezkel 18:10 and 33:30; Tehillim 49:8; Daniel 2:31, 3:19, 6:3, 7:5, 7:16; Megillah 28a; Sanhedrin 93b; Bamidbar Rabbah 18:10; Tanchuma Korach 7; Bereishis Rabbah 98:13; Radak, Sefer HaShorashim and Michlol; Ibn Janach, Sefer HaShorashim; Rabbi Wolf Heidenheim as quoted by HaKtav VaHaKabbalah to Bamidbar 16:15; Rabbi Shlomo Zalman Hanau, Shaarei Tefillah 11; Malbim, Ayelet HaShachar introduction to Vayikra 69 and Artzos HaChaim Orach Chaim 6:1; Rabbi Shmuel Yehudah Steiger, Avnei Shayish; Rabbi Yehonosson Eybeschutz on Adam's language.

Restored in this pass: the machlokes over Bamidbar 16:15 between Rashi with Targum Onkelos and Rashbam with Chizkuni and Ramban, which is the question Heidenheim's two forms are answering, and Rabbi Yehonosson Eybeschutz on the aleph of echad. All five of those authorities stood in the front matter with nothing in the body attached to them.

The entry has no words: block, and so no pointed forms for אחד, אחת or חד. That is left for whoever maintains that block; the entry stays draft on account of it, not on account of its content.