The term “principle parts” actually refers only to verbs, but we can extend it to nouns for the moment. For verbs, you can tell which conjugation a verb is by looking at its second principle part, the infinitive. Verbs with -ā re are first conjugation, -ē re are second, and so on. Latin nouns are classified into declensions based on how they form their genitive case.

The genitive case is the second one down on the charts, right after the nominative. When you look a noun up in the dictionary, or when you learn your vocabulary, you will find dictionary entries that look like this:

Each entry provides you with four pieces of information, and they are all you need to be able to work with or understand that noun. First is the nominative case form. This is especially important for third declension words since there is no regular, predictable nominative ending for third declension words. The second item is the genitive singular form. The third is an abbreviation referring to the word's gender. The fourth bit of information is meaning and usage.

The second item, the genitive form, is extremely important. It tells you two crucial things. First, it says what declension the word belongs to. Their nominative forms or genders don't play a role in that decision. Here is a breakdown of how to tell them apart.

• All nouns with a genitive in -ae are first declension.

• All nouns with a genitive in -ī are second declension.

• All nouns with a genitive in -is are third declension.

• All nouns with a genitive in -ūs are fourth declension.

• All nouns with a genitive in -ēī are fifth declension.

In addition to signaling declension, the genitive form shows you whether the word has a stem change, and if it does, what that change is.

In the list of words given above, you know terra is first declension because its genitive form is -ae (or terrae once you attach the ending). Deus is second declension and becomes deī when you decline it. Caelum is also second declension. The word rē x is third declension. The second form for rē x tells you that it is third declension because it ends in -is, and that rē g- is the stem that all the endings will go on.