This das is not a definite article for a neuter noun (after all, there is no noun). This is a word that means "it, this, that". It points the attention to a specific object.
eine is the indefinite article for feminine nouns in Nominative.
Mädchen means girl and that word is surprisingly a neuter noun, not feminine. Therefore, the "ein" in front of it.
This is just an example of why it is important to learn the gender of the noun together with the noun.
Remember, the adjective is behind the noun, so the adjective doesn't change.
The adjective is in front of the noun and functions as an attribute. Adjectives in this case are declined, so "blau" becomes "blaues" because Pferd is a neuter noun.
Key phrases
Was ist das?
Wer ist das?
Wie alt ist dein Bruder?
Mein Bruder ist drei Jahre alt.
Warum ist das Pferd blau?
Questions
1. _ _ _ ? alt ist dein Auto? 2. _ _ _ _ _ ? bist du hier? 3. _ _ _ ? Mädchen ist sehr schön. 4. _ _ _ ? Tasche ist so schön. 5. Meine Tasche ist nicht im _ _ _ _ ?.