Der Winter ist eine Jahreszeit.

Because Winter is a masculine noun in German. German nouns have grammatical gender, so you learn them together with their article:

• der Winter (masculine)
• die Jahreszeit (feminine)

In this sentence, der is the nominative masculine singular definite article, because Winter is the subject of the sentence.

In German, all nouns are capitalized. So both Winter and Jahreszeit begin with a capital letter.

This is one of the most noticeable spelling rules in German:

• der Winter
• eine Jahreszeit

Because Jahreszeit is a feminine noun: die Jahreszeit.

The indefinite articles in the nominative are:

• ein for masculine and neuter
• eine for feminine

So:

• ein Winter would be wrong here
• eine Jahreszeit is correct

Both nouns are in the nominative.

That is because:

• Der Winter is the subject
• After sein (ist), the noun that identifies or describes the subject is also usually in the nominative in German

So:

• der Winter = nominative
• eine Jahreszeit = nominative

This is similar to English in sentences like Winter is a season.

German main clauses usually follow the verb-second rule. That means the finite verb comes in position 2.

In this sentence:

• Position 1: Der Winter
• Position 2: ist
• Then: eine Jahreszeit

So the basic pattern is:

Subject + verb + complement

Because the subject, der Winter, is singular.

The verb sein changes depending on the subject:

• ich bin
• du bist
• er/sie/es ist
• wir sind
• ihr seid
• sie/Sie sind

Since der Winter is singular, you use ist.

Yes, it is one word. German often forms compound nouns.

Jahreszeit is made from:

• Jahr = year
• -es- = linking element
• Zeit = time

So literally it is something like year-time, but the actual meaning is season.

This is very common in German. Many long nouns are built by combining smaller words.

Usually, no. In standard German, you normally say Der Winter ist eine Jahreszeit.

German often uses an article where English may not focus on one. Since Winter here is a specific noun used as the subject, the definite article der is natural and expected.

Without the article, it would sound incomplete or unnatural in this sentence.

A simple English-friendly guide would be:

YAH-res-tsite

More roughly:

• Jah- sounds like yah
• -res- has a light r
• Zeit sounds like tsite or tsait

The z in German is pronounced ts.

So the full sentence sounds roughly like:

dair VIN-ter ist EYE-nuh YAH-res-tsite

Yes. It is a simple definition / classification sentence.

It says that Winter belongs to the category Jahreszeit.

German often uses this pattern for definitions:

• Ein Hund ist ein Tier.
• Eine Rose ist eine Blume.
• Der Winter ist eine Jahreszeit.

So this is a very useful structure to learn.