Learn Hardcore Russian: Облака плывут по синему небу.

The preposition по is used to indicate movement over or along a surface. In this sentence, it shows that the clouds are drifting across the blue sky. Additionally, по governs the dative case in such constructions, which is why the noun appears as небу and its accompanying adjective as синему.

Why is the adjective синий modified to синему instead of remaining in its nominative form синее?

Even though небо (sky) is a neuter noun and its nominative form is синее, the preposition по requires the noun to be in the dative case. In the dative, the neuter form of the adjective синий becomes синему to agree in gender, number, and case with небу.

What tense and aspect is the verb плывут, and what does it suggest about the action?

Плывут is the third person plural form of the verb плыть in the present tense. It is in the imperfective aspect, which emphasizes an ongoing or continuous action. In this context, it vividly describes the gentle, continuous drifting of the clouds.

Why is the noun облака presented in the plural form?

The noun облака is the plural form of облако (cloud). Using the plural implies that there are multiple clouds drifting in the sky, creating a more dynamic and collective image of the scene rather than focusing on just one cloud.

Could the preposition по be replaced with на in this sentence? If so, what nuance would that change introduce?

While на could be used, as in на синем небе, this would mainly indicate the location of the clouds. The use of по emphasizes their movement across the sky. Changing the preposition to на would shift the focus from the clouds’ drifting action to merely stating where they are, thereby altering the dynamic imagery of the sentence.