And also the number אחד (one) will come in construct cardinal declension and ordinary cardinal declension - אֶחָד. אַחַד – when it comes in construct cardinal declension, the letter מ will sometimes come after it - כנפש אַחַד מהם (as the life of one of them)¹⁴⁰, and sometimes the definite article ה will come after it - אַחַד ההרים(one of the mountains)¹⁴¹.

footnotes: ¹⁴⁰ 1 Kings 19:2. ¹⁴¹ Genesis 22:2.

And the grammarians write that אֶחָד and אַחַד are of two declensions:

אֶחָד is of heavy declension like סַבָּל ,שַבָּת ,גחָל ,פֶּחָם.

And אַחַד is of light declension like נַחַש ,שַחַת.

And indeed, sometimes the noun אַחַד is not a declension on its own, but only a noun that comes in construct state that is derived from the noun אֶחָד. And this is when the definite article ה comes afterwards like אַחַד העם (one of the people),אחד ההרים (one of the mountains). Only if the prefix מ comes afterwards will it not come in construct state, and then it will definitely be of light declension.

And with this stand the differences between אֶחָד of heavy declension and אַחַד of light declension: When אֶחָד comes, it intends to exclude duality, and it means to say “one and not two”. And with the noun אַחַד, it does not intend to exclude duality, only that it is a noun that indicates generality, and it means to say “any one that will be”. And for the noun אַחַד in which the definite article ה comes afterwards and is derived from heavy declension and comes in construct state, its rule is like that of the nouns שלושת (third), ארבעת (fourth) that I mentioned in the previous rule, which come to indicate that it is a known number. And thus, when it says אַחַד ההרים, it means to say it is a known mountain, which is Mount Moriah; אחד העם (one of the people) – is the king, etc.