Questions & Answers about Девочка мечтает о новом телефоне.
Why do we use о here instead of об?
The preposition for "about" changes depending on the first letter of the word immediately following it. We use о before words that start with a consonant, like новом. We use об before words that start with a vowel to make them easier to pronounce.
Why does the word for phone end in -е (телефоне)?
Because it follows the preposition о ("about"), which requires the noun to be in the Prepositional case. For most masculine nouns that end in a consonant in their dictionary form, like телефон, you simply add -е to the end.
Why is the adjective новом instead of its dictionary form новый?
Adjectives must match the noun they describe in gender, number, and case. Because телефоне is masculine and in the Prepositional case, the adjective describing it must also take the masculine Prepositional form. The ending -ый changes to -ом.
Does мечтает mean dreaming while asleep?
No. The verb мечтать means to daydream, wish, or strongly hope for something, just like this girl wanting a new phone. To talk about seeing a literal dream while sleeping, Russians use a completely different phrase involving the word сон.