Der fünfte Tag ist ruhig.

The fifth day is quiet.

Breakdown of Der fünfte Tag ist ruhig.

• sein: to be
• der Tag: the day
• ruhig: quiet
• fünfte: fifth

Questions & Answers about Der fünfte Tag ist ruhig.

Why is Tag capitalized in German?

In German, all nouns—common and proper—are capitalized. Since Tag (“day”) is a noun, it always begins with a capital letter.

Why is the article der used here instead of die or das?

German nouns have grammatical gender. Tag is a masculine noun, so in the nominative case (as the subject of the sentence) it takes the masculine definite article der.

Why does fünfte end in -e and not -er or -es?

Fünfte is an ordinal adjective placed before a noun and following a definite article. In this setup—masculine, singular, nominative with a “der” article—adjectives use the weak declension ending -e.
Scheme: der (article) + fünfte (weak ending) + Tag (noun).

Why doesn’t the predicate adjective ruhig take any ending (e.g. ruhige, ruhiger)?

When an adjective follows a linking verb like ist (the copula “to be”), it’s called a predicate adjective. Predicate adjectives in German remain in their base form (no declension) because they describe the subject rather than directly modify a noun with an article.

Why is ist in the second position in the sentence?

German main clauses almost always follow the V2 rule: the finite verb (here ist) must occupy the second position.
1st position: Der fünfte Tag (subject)
2nd position: ist (verb)
remainder: ruhig (predicate adjective)

What case is der fünfte Tag in, and how do we know?

It’s in the nominative case, because it’s the subject of the sentence. Clues:

• The verb ist (sein) takes a nominative subject.
• The article der is the nominative masculine singular form.

Could I also say Am fünften Tag ist es ruhig? What would change grammatically?

Yes. This version uses a temporal prepositional phrase: am fünften Tag = “on the fifth day.”
Changes:

• am = an + dem, so dem requires dative.
• The adjective fünften takes the dative ending -en.
• You add the dummy pronoun es as the grammatical subject for ist.
  Full breakdown:
  “Am fünften Tag” (time in dative) + “ist” (V2) + “es” (subject placeholder) + “ruhig” (predicate adjective).