Ren in corpore est.

Because in is showing location here: in the body, not movement into the body.

In Latin, in takes:

• the ablative for location: in corpore = in the body
• the accusative for motion toward: in corpus = into the body

The noun is corpus, corporis, so its ablative singular form is corpore.

Ren is nominative singular. It is the subject of est, so it is the thing that is somewhere.

The dictionary form is ren, and the full entry is ren, renis.

Corpore is ablative singular.

That is because the preposition in, when it means in/on/within in the sense of location, normally takes the ablative.

So:

• ren = subject, nominative
• corpore = object of the preposition in, ablative

Latin does not have articles like English the and a/an.

So ren can mean:

• a kidney
• the kidney
• sometimes just kidney in a general sense

You figure out which one is meant from the context.

Latin word order is much more flexible than English word order.

Putting the verb at the end is very common in Latin prose, so Ren in corpore est is perfectly normal.

Other word orders are also possible, for example:

• Ren est in corpore
• In corpore ren est

These all mean basically the same thing, though the emphasis may shift slightly.

The usual dictionary forms are:

• ren, renis = kidney
• corpus, corporis = body
• sum, esse = to be

From those forms, you get:

• ren = nominative singular
• corpore = ablative singular
• est = he/she/it is

Yes.

• ren, renis is a third-declension masculine noun.
• corpus, corporis is a third-declension neuter noun.

This is one reason why learning the genitive singular is so important in Latin. The nominative alone often does not tell you enough about how the noun declines.

Because many third-declension nouns form the ablative singular with -e.

For corpus, corporis:

• stem: corpor-
• ablative singular ending: -e

So:

• corpor- + e = corpore

That is the form needed after in for location.

Yes, but it would mean something different.

• in corpore = in the body / inside the body
• in corpus = into the body

So in corpus would be used if there were motion toward the inside of the body.

• ren is masculine
• corpus is neuter

In this sentence, gender does not show up very clearly because there is no adjective agreeing with either noun. But gender still matters in Latin, because adjectives, pronouns, and some participles must agree with the noun they describe.

In a classical pronunciation, you can say it approximately as:

ren in COR-po-re est

A few helpful points:

• c in corpore is always hard, like k
• e is a pure vowel, not the English diphthong sound
• r is ideally trilled or tapped
• each vowel is pronounced clearly: cor-po-re

Sometimes Latin can omit forms of to be, especially in poetry, headings, notes, or very compressed style. But for a normal beginner sentence like this, you should keep est.

So the standard full sentence is:

Ren in corpore est.

The plural would be:

Renes in corpore sunt.

That means the kidneys are in the body or kidneys are in the body, depending on context.

Here:

• renes = nominative plural of ren
• sunt = are

The phrase in corpore stays singular if you mean in the body as one body.