Learn Russian: Ранним утром улицы пустые. - Early in the morning the streets are empty.

In Russian, to state that a general state or action happens during a certain part of the day, you use the Instrumental case without a preposition. The neuter noun утро (morning) becomes утром. The adjective ранний (early) must agree with it, so it also takes the Instrumental case, becoming ранним.

Why does ранним end in -им instead of -ым?

The dictionary form is ранний, which is a 'soft' adjective. Because its stem is soft, it takes the Instrumental neuter ending -им instead of the standard hard ending -ым that you might be used to seeing on other adjectives.

Can I use the phrase рано утром here instead?

Yes, you can! Рано утром uses the adverb рано (early) alongside the time expression утром. Ранним утром uses the adjective ранний directly attached to the noun утро. Both translate to 'early in the morning,' but ранним утром is slightly more descriptive.

Why are there no verbs in this sentence?

This sentence describes a state in the present tense. In Russian, the present tense forms of the verb 'to be' (am, is, are) are almost always omitted. You simply put the subject (улицы) right next to the adjective (пустые).