Мой телефон дороже, чем книга.

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Breakdown of Мой телефон дороже, чем книга.

• книга: the book
• телефон: the phone
• чем: than

Questions & Answers about Мой телефон дороже, чем книга.

Why is дороже used here instead of более дорогой?

Russian has two ways to form comparatives of adjectives:

• Synthetic comparatives (one-word forms): дороже, моложе, старше, быстрее
• Analytic comparatives (two-word forms with более/менее): более дорогой, менее дорогой

For most common adjectives—especially those with one or two syllables—the synthetic form (дороже) is more idiomatic and concise. The analytic form (более дорогой) exists and is perfectly correct, but it sounds heavier in everyday speech.

How is дороже formed from the adjective дорогой?

Morphologically, дорогой → дороже is formed by:

1. Dropping the ending -ой
2. Softening the stem consonant г → ж (a regular phonetic change)
3. Adding the comparative ending -е

So дорог- + ж + е = дороже.

Why is there a comma before чем?