There are three grammatical genders, which follow very closely actual gender. The following rules apply:
| Masculin | Feminin | Neutral |
| Male human beings, animals masculine professional names, trees, plants and seeds, rivers, mountains, lakes, seas numbers, letters, notes, colors, months, days, gender specific groups of humans, animals, etc… |
Female human beings, animals feminine professional names, fruits, and flowers, cities, regions, nations, islands ships, cars, trains, airplanes, gender specific groups of humans, animals, etc… |
Inanimate and man made objects, groups of humans, trees or animals, parts of living organisms materials, minerals, events and activities, abtract concepts, natural phenomena, generic groups of humans, animals, etc… |
| rege, leon, actor, olivo, rosario, frumento, Rheno, Monteblanco, Erie, Mediterraneo, lo tres, lo zed, lo sol, lo rubio, lo lunedi, li richi, li leoni, li italiani, etc… |
regina, leonessa, actrice, oliva, rosa, spica, Roma, Nova Angleterra, Australia Titanic, Orient-Express, le riche, les leonesse, le italiane, etc… |
petra, casa, cannon, satellite, armea, tribo, oliveto, grege, mano, corde, ramo, ala, corio, plumbo, esmeraldo recolto, le pensar, concordia, confusion, altruismo, timor, pluvia, les riches, les leones, les italianes, etc… |
Foreign and loan words tend to maintain the same gender as in the language of origin. Gods, planets, moons, satellites, constellations, etc… have the same gender as the original Græco-Roman equivalent. Abstract concept (if personified) also have the same gender as the original Græco-Roman equivalent and are feminine when ending in -a/-ion/-ate, masculine when ending in -o/-or.
For purposes of declension, Interlingua nouns, articles, pronouns and adjectives can be divided into three declension, of which the third is mostly used for foreign loan words.
| I Declension | Masculin | Feminin | Neutral |
|---|---|---|---|
| ending = o,a,n | ending = a | ending = a,o | |
| Direct Case Singular | puero - artista | puera - artista | casa - mano |
| Indirect Case Singular | pueru - artista | puera - artista | casa - manu |
| Vocative (Honorific) Singular | Paulé - pueré - artista | Julié - pueré - artista | casa - manu - Romá |
| Direct Case Plural | pueri - artisti | puere  - artiste  | casas - manos |
| Indirect Case Plural | pueros - artistas | pueras - artistas | casas - manus |
| Vocative (Honorific) Plural | pueri - artisti | puere - artiste | casé - manua - Romá |
| II Declension | Masculin | Feminin | Neutral |
|---|---|---|---|
| ending = e,c,l,n,r,s,t | ending = e,c,l,n,r,s,t | ending = e,c,l,n,r,s,t | |
| Direct Case Singular | rege - leon | matre - limon | nomine - cannon |
| Indirect Case Singular | rege - leone | matre - limone | nomine - cannone |
| Vocative (Honorific) Singular | rege - leone - lectó | matre - limone - lectrí | nomine - cannone - Anglé |
| Direct Case Plural | leoni - regi | matri - limoni | nomines - cannones |
| Indirect Case Plural | leones - reges | matres - limones | nomines - cannones |
| Vocative (Honorific) Plural | regi - leoni - lectori | matres - limones - lecricres | nomines - cannones - Anglé |
| III Declension | Masculin, Feminin | Neutral | Masculin, Feminin, Neutral |
|---|---|---|---|
| ending = i, u, any accented vowel | ending = i, u, any accented vowel | ending = loan words | |
| Direct Case Singular | attaché - fiancée | safari - tabu - taxí | album |
| Indirect Case Singular | |||
| Direct Case Plural | attachés - fiancées | safaris - tabus - taxís | |
| Indirect Case Plural |
The moderne endings "ki", ke, kes" are preferred over and should replace "chi, che, ches".
The Vocative can be preceeded by the interjections "O" or "OH", and are associated with the "Tu" or with the "Vu":
The moderne endings "ki", ke, kes" are preferred over "chi, che, ches".
| I Declension | Masculin | Feminin | Neutral |
|---|---|---|---|
| ending = e,i,c,n,r,s,t + all monosyllables | ending = a,i,c,n,r,s,t + all monosyllables | ending = e,i,c,n,r,s,t + all monosyllables | |
| Direct Case Singular | bon - rustic - horari - parve | bon - rustic - horari - parve | bon - rustic - horari - parve |
| Indirect Case Singular | bono - rustico - horario - parvo | bona - rustica - horaria - parva | bone - rustiche - horarie - parve |
| Direct Case Plural | boni - rustichi - horarii - parvi | bone - rustiche - horarie - parve | bones - rustiches - horaries - parves |
| Indirect Case Plural | bonos - rusticos - horarios - parvos | bonas - rusticas - horarias - parvas | bones - rustiches - horaries - parves |
| II Declension | Masculin | Feminin | Neutral |
|---|---|---|---|
| ending =e,l,ce,le,ne,se,te | ending =e,l,ce,ne,le,se,te | ending =e,l,ce,le,ne,se,te | |
| Direct Case Singular | habile - final - feroce - cortese | habile - final - feroce - cortese | habile - final - feroce - cortese |
| Indirect Case Singular | habile - finale - feroce - cortese | habile - finale - feroce - cortese | habile - finale - feroce - cortese |
| Direct Case Plural | habili - finali - feroci - cortesi | habili - finali - feroci - cortesi | habiles - finales - feroces - corteses |
| Indirect Case Plural | habiles - finales - feroces - corteses | habiles - finales - feroces - corteses | habiles - finales - feroces - corteses |
| III Declension | Masculin, Feminine | Neutral | Masculin, Feminine, Neutral |
|---|---|---|---|
| ending = i, u, any accented vowel | ending = i, u, any accented vowel | ending = loan words | |
| Direct Case Singular | tabú | tabu | kasher(kaxer) |
| Indirect Case Singular | |||
| Direct Case Plural | tabús | tabús | |
| Indirect Case Plural |
Adjectives always use only the Nominative as Vocative/Honorific.
Definite and indefinite articles and possessive pronouns belong to the first declension.
| ending = o | ending = a | ending = e | |
| Direct Case Singular | el, l' - un - mi | el, l' - un - mi | el, l' - un - mi |
| Indirect Case Singular | lo - uno - mio | la - una - mia | le - une - mie |
| Direct Case Plural | li - uni  - mii | le - une  - mie | les - unes  - mies |
| Indirect Case Singular | los - unos  - mios | las - unas  - mias | les - unes  - mies |
| Gender | Subject | Genitive | Dative | Accusative - Reflexive |
Possessive I Adjectives |
Possessive II Adjectives |
|---|---|---|---|---|---|---|
| I Person Singular Masculine / Neutral | io | mi | mei | me | mi | mio - a - e |
| I Person Singular Feminine | ia | mi | mei | me | mi | mio - a - e |
| II Person Singular Masculine / Neutral | tu | tu | ti | te | tu | tuo - a - e |
| II Person Singular Feminine | tua | tu | ti | te | tu | tuo - a - e |
| II Person Singular Honorific (Masc. & Fem.) | Vu | vu | vui | ve | vu | vuo - a - e |
| III Person Singular Mascuilne | esso | su | lui | se | su | suo - a - e |
| III Person Singular Feminine | essa | su | lei | se | su | suo - a - e |
| III Person Singular Neutral | esse | su | le | se | su | suo - a - e |
| III Person Indefinite (Person) | on | son | soi | se | son | n/a |
| III Person Indefinite (Object) | il | n/a | n/a | n/a | n/a | n/a |
| I Person Plural Masculine / Neutral | noi | nos | ne | nos | nos | nostro - a - e |
| I Person Plural Feminine | noe | nos | ne | nos | nos | nostro - a - e |
| II Person Plural Masculine / Neutral | voi | vos | ve | vos | vos | vostro - a - e |
| II Person Plural Feminine | voe | vos | ve | vos | vos | vostro - a - e |
| II Person Plural Honorific Masculine | Vui | ves | vue | ves | ves | vestro - a - e |
| II Person Plural Honorific Feminine | Vue | ves | vue | ves | ves | vestro - a - e |
| III Person Plural Masculine | essi | lor | los | se | lor | loro - a - e |
| III Person Plural Feminine | esse | lor | las | se | lor | loro - a - e |
| III Person Plural Neutral | esses | lor | les | se | lor | loro - a - e |
|   | Nominative | Genitive | Dative | Accusative |
|---|---|---|---|---|
| Masculin & Feminine | qui | cuye | cui | que |
| Neutral | que | de que | a que | que |
| Indeterminate | le que | de que | a que | el que |
| Adjectival | lo, la, le qual li, le quali - les quales |
del qual del quales |
al qual al quales |
el quale los, las, les quales |