Latin · Chapter 2 · Lesson 4 · Grammar primer
Declensions: why the dictionary gives two forms
rex, regis. corpus, corporis. The second form is the only reliable way to know what endings a noun takes - and often the only way to find its stem.
The short version
Learn the genitive with the noun. Always.
Every dictionary prints two forms, rex and regis, and the second is not decoration. Its ending tells you which of the five declensions the noun belongs to and therefore what endings it takes. Its stem tells you what every other form is built on. A noun learned without its genitive is a noun you can recognise and cannot use, which is why this is the most repeated advice in Latin teaching.
rex, regis
The third declension hides its own stem
This is the whole argument for learning the genitive. You could not have guessed any of these from the nominative: corpus becomes corpor-, nomen becomes nomin-, iter becomes itiner-, homo becomes homin-, rex becomes reg-. The third declension is the biggest and holds a striking share of the commonest nouns, and all of its difficulty sits in the nominative singular.
corpus, corporis; iter, itineris
Neuter nouns look the same doing and receiving
Every neuter noun in Latin has a nominative identical to its accusative, and in the plural both end -a. Bellum is war whether it is the subject or the object, and bella is wars either way. That saves you a good deal of memorisation and costs you the ability to tell subject from object on a neuter noun, which context has to settle instead.
bellum, bella
That's the gist. The full article goes deeper - more examples, the edge cases, the why - and the practice below scores like a vocabulary session and earns the same XP.
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