Ecclesiastical Latin/NominativeNoun

The nominative case is the case used in Latin to show the subject of a sentence. It is also the base form of most words. As such, nominative have wide array of forms. Nouns in Latin will usually fall into one of five "declentions", or groups, depending on how the sounds in the word change to show a different grammatical function in the sentence. However, it is almost impossible to tell from just the nominative form which group the word belongs to. It is also difficult in some situations to know what gender the word is.

These difficulties are remedied in the following ways;

Know your vocabulary. If you know the word, and all its forms, then you have no problem. If you don't know the word but know many words like it, then you can also make a very good guess.
Know what the genitive looks like. We are not studying the genitive case yet, but if you know what the genitive looks like, then you know which group the word goes in. The dynamic between the Nominative and the Genitive is how we are able to make the groups in the first place.
Get your dictionary. The nominative is the form used to list words in the dictionary. In the dictionary, you will see the genitive form as well, telling you the group, and the gender.

Latin nouns normally are classed into five different groups. Three of these groups contains words that have gender and words that do not have gender. Latin words can be both singular and plural and the way to form the plural differs according to group and gender. The basic grammatical rule for forming the plural is to take the genitive case and remove the genitive ending and replace the genitive ending with the plural ending. This can be confusing for the beginner student so simplified example follow.

AE group words can be masculine or feminine but most by far are feminine. They can not be neuter. Neuter words have no gender but all AE group words have gender. The rule for forming the Plural in this group is to add the E sound to the word.
Puella - Puellae
I group words can be masculine or neuter but cannot be feminine. The masculine words follow one rule but the neuter words follow a different rule. Masculine words add I to the end of the word but neuter words add A.
Masculine words
Puer - Pueri
Servus - Servi
Vir - Viri
Magister - Magistri
Sometimes the masculine word just adds I and sometimes the original word has to change to add I.
Neuter words
Regnum - Regna
In this group all neuter words replace the UM with A.
IS group words can be masculine, feminine or neuter. This is the largest group of words in the Latin language. All of words with gender add ES to the end of the word and all neuter words add A to the end. Sometimes the original word just adds ES but most of the time the original word has to change so that ES can be added. Some words will appear to have no change if they already end in ES.
Gender words
Mulier - Mulieres
Homo - Homines
feles - feles
clavis - clavis
Urbs - Urbes
Neuter words
corpus - corpora
animal - animalia

US group words are few in number. Most are masculine but a small number a feminine or neuter. The gender words have no change in order to form the plural. The neuter words add A to the end.

Gender words
fructus - fructus
manus - manus
neuter words
genu - genua

EI group words are almost all feminine. Only the word DIES is masculine and that is only half the time. EI group words can no not be neuter. These words have no change in forming the plural.

res - res
dies - dies

New Words

New words come into Latin all the time and one of the things people think and argue about is which group to put it in. This for us really is needless. As far as nominatives go, just say the word with nothing added to it and that can function perfectly well as the nominative case. For other cases, if you follow the rules of Group Three, everyone will understand.

Keep in mind, many words like Domus, will jump from one group to another over time, or depending on who is saying the word, so don't worry about it.