The endings יִם and וֹת – for the simplest noun type
Nouns mark their plural by the endings יִם and וֹת . Nearly all masculines
take יִם and nearly all feminines וֹת .
Before adding ים , the masculine noun first drops any ֶה or ִי ending it has.
And before adding וֹת , the feminine noun first drops the singular feminine
ending ָה or ִת . Thus:
יְהוּדִי - Jew ; יְהוּדִים - Jews
ילדה - girl ; ילדות - girls
There are some exceptions, e.g. the masculine noun שְׁוּלְחָן - ‘table’ has the
plural שְׁוּלְחָנוֹת . Conversely, feminine שָׁנָה ‘year’ has the plural שָׁנִים .
The form that a noun happens to take in the plural has no effect on its
intrinsic gender. Thus שְׁוּלְחָנוֹת ‘tables’ is as masculine as שְׁוּלְחָן ‘table’,
and hence the agreement שְׁוּלְחָנוֹת מַקְסִימִים ‘gorgeous tables’.
To make the plural of a noun is often more than just a matter of adding
an ending: the internal vowels may have to be changed, depending on the
form of the word. Obviously, this generally affects pronunciation rather
than spelling, as Hebrew is mostly written without vowels.
When the vowel a becomes third vowel from the end (thanks to the
presence of an ending), many nouns omit it. Thus:
דבר (davar - thing) ; דברים (dvarim - things)
גָמָל (camel) ; גְמַלִים (gamal; gmalim)
However, many nouns do not observe this rule. 1 Either they begin with one of the four letters ע , ח , ה , א(so-called ‘guttural’ letters), which for phonetic reasons require the acoustic ‘support’ of a full vowel;
2 or the a has the vowel point ָ rather than ַ (which again for historical reasons could not drop). E.g. חָכָם ; חֲכָמִים (sage; sages).
To know if a noun has ָ or ַ is a matter of recognizing characteristic patterns – or consulting a dictionary.