1. If you really want to be a taxonomist (someone who classifies species and describes species) you should take Latin courses, because the rules of Latin grammar differ greatly from those of English grammar. When you describe a new animal species, you must make the name conform to Latin grammar, but you may describe it in English. When you describe a new plant species, you must write the entire description and the name in Latin.
2. The second word of a species name is (basically) an adjective in Latin, but there are exceptions (SEE para 5 below). If it is an adjective, it must agree in number, case, and gender with the generic name.
3. Adjectives in Spanish have one case, two numbers (singular and plural), and two genders (masculine and feminine) to give four possible endings (SECTION 13). German adjectives have 24 possible endings (two numbers, three genders, and four cases). Although Latin adjectives have 36 possible endings (six cases, two numbers, and three genders [masculine, feminine, and neuter]), you have to consider only one case (the nominative), one number (the singular), and three genders in selecting Latin adjectives to form species names.
4. To select a Latin adjective to form a species name, you have to know what is the gender of the generic name. The adjective has to match the generic name in gender. The describer of a new genus should give its gender. If he/she does not do so, you have to determine it from its etymology, from the name of the type species, or from other species names included: to do this takes a knowledge of Latin grammar. Note that some adjectives used in species names are verbal adjectives, whose endings differ from those of "simple" adjectives. Others are formed from geographical names.
5. However, in some names, the specific epithet is formed not from an adjective, but from a noun. Nouns have 6 cases in the singular (and 6 in the plural). Typically, a noun is used in the genitive case (SEE PARA 7) in the singular or sometimes in the plural, or in the nominative case (the first of the 6 cases) "in apposition." Nouns do not have just one set of endings but are arranged into 5 major groups (called declensions), within which endings differ by number (2) and case (6), and exceptions exist. As examples, the genitive cases (singular) of the Latin nouns terra, liber, urbs, portus, and nomen are (respectively) terrae, libri, urbis, portus, and nominis. To prescribe new scientific names takes a knowledge of Latin. But, Latin is just a matter of following the rules - millions of people have been able to do that.
6. The rules for naming new species may be greatly simplified in forthcoming editions of the International Code of Zoological Nomenclature.
7. Nouns have cases in Latin and German, but not in French, Italian, Spanish, or Portuguese. One of these cases is the genitive (sometimes called possessive): it is the 3rd case in German (of 4) and the 4th in Latin (of 6). The genitive case exists in English! It means of something or someone or somewhere. Thus, In English we can write "cat's eye" -- by adding "'s" to a noun such as cat, we form the genitive case. That cannot be done in French, Italian, Portuguese, or Spanish, so that in those languages we have to write "eye of cat" with less convenience.