Die Puppe ist alt.

In German, all nouns are capitalized, not just proper names.

So:

• die Puppe = the doll
• das Haus = the house
• der Mann = the man

This is one of the easiest visual clues in German for spotting nouns.

Here, die is the definite article meaning the.

In this sentence, die is:

• feminine
• singular
• nominative (because Puppe is the subject of the sentence)

So die Puppe means the doll.

You know it is feminine because it uses the article die.

In German, every noun has a grammatical gender:

• der = masculine
• die = feminine
• das = neuter

So when you learn a noun, it is best to learn it together with its article:

• die Puppe, not just Puppe

Also, many nouns ending in -e are feminine, though not always. So Puppe being feminine is not surprising.

German main clauses usually follow the verb-second rule, often called V2.

That means the conjugated verb comes in the second position:

• Die Puppe ist alt.

Here:

1. Die Puppe = first element
2. ist = second element
3. alt = rest of the sentence

This is very normal German word order for a basic statement.

Sein is the infinitive, meaning to be.
Ist is the conjugated form used with er / sie / es and with singular nouns like die Puppe.

Conjugation of sein in the present tense:

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

Because die Puppe is a singular noun, the correct form is ist.

Because alt comes after the verb sein and works as a predicate adjective.

In German:

• adjectives before a noun usually take endings
• adjectives after sein, werden, or bleiben usually do not take endings

So:

• die alte Puppe = the old doll
• Die Puppe ist alt. = The doll is old.

That is why it is just alt, not alte.

They are related, but they do different jobs.

• Die Puppe ist alt.
  = a full sentence
  = The doll is old.

• die alte Puppe
  = a noun phrase, not a full sentence
  = the old doll

In die alte Puppe, the adjective comes before the noun, so it takes the ending -e.
In Die Puppe ist alt, the adjective comes after ist, so it has no ending.

They are completely unrelated.

In German, die is pronounced roughly like dee.
It means:

• the for feminine singular nouns
• also the for plural nouns in some contexts

It is not pronounced like the English verb die.

In general, yes: die can also be the plural form of the.

But in this sentence, it is clearly feminine singular, because:

• Puppe is singular
• the verb is ist (singular), not sind (plural)

Compare:

• Die Puppe ist alt. = The doll is old.
• Die Puppen sind alt. = The dolls are old.

A simple pronunciation guide is:

Die Puppe ist alt.
dee POOP-peh ist alt

A few notes:

• die sounds like dee
• Puppe has two syllables: Pup-pe
• ist is short and clear
• alt rhymes roughly with English halt without a strong h

The stress is mainly on the first syllable of Puppe: PUP-pe.

Normally, no. In standard German, singular countable nouns usually need an article or some other determiner.

So the normal sentence is:

• Die Puppe ist alt.

Without the article, Puppe ist alt sounds incomplete or unnatural in ordinary German.

It can describe both.

With people, alt usually means old in age.
With things, it can mean:

• old in age
• old as opposed to new
• aged, older, or not new

So Die Puppe ist alt can naturally mean that the doll is old, perhaps because it is an old toy or not new anymore.