Bulgarian nouns have the categories: grammatical gender, number, case (only vocative) and definiteness.
In most cases the gender of the noun can be determined according to its ending, but there aren't any strict rules.
Nouns referring to people or animals can change their gender (from masculine to feminine) by adding the suffixes: -ка, -ица, -а, -иня: The gender of nouns that have no singular form can't be determined: финанси, очила, обуща.
Generally if the noun ends in a vowel, it is removed before adding the plural suffix.
An exception to this occurs in some exclamations following колко, when the ordinary plural is used and the inferred meaning is "what a large amount of!
There are also a handful of neuter nouns for parts of the body which also take a count form – ramo (shoulder) → ramene / 2 ramena; kolyano (knee) → kolene / 2 kolena.
When "литър" (litre) and "метър" (metre) mean measures of volume and length, their count form is "литра" and "метра" respectively.
But when "метър" means "meter" (a device that measures and records the amount of electricity, gas, water, etc.)
For example: кълбо — кълба and кълбета, крило — крила and криле, рамо — рамена and рамене, коляно — колена and колене, море — морета and моря, дърво — дървета, дърва and дървеса, четене — четения and четенета.
Some plural forms have different meanings: дърва – fuel wood, дървета – trees; some are used in specific contexts: the variants моря (from море), поля (from поле) are found only in poetry.
This category includes some abstract nouns (материализъм – materialism, сигурност – security, любов — love); some collective nouns (студентство – students); and chemical elements and some other substances (водород – hydrogen, въглерод – carbon, грис – semolina, ориз – rice).
These are nouns referring to objects composed of two identical parts (очила – glasses, ножици – scissors), and some concepts and objects consisting of many elements (въглища – coal, финанси – finances, пари – money).
There is an additional discrepancy between pronunciation and spelling when the stress of some monosyllabic masculine nouns falls on the definite articles -а or -ят/я.
The former are used always, when addressing someone, and the use of nominative forms instead is immediately perceived as a gross error: The latter, however, are considered informal, and are used less frequently, especially the vocative forms of female personal names ending in -о, which are even considered by some to be rude or rustic (Елено, Богдано).
Nevertheless, nominative forms (especially the male ones) sound too formal, even snobbish, and are used rarely by native speakers.