Learn Finnish: Pihalle leijailee aavemaista usvaa. - Ghostly mist floats into the yard.

This is a dynamic existential clause. Instead of the static verb olla (to be), it uses a dynamic verb of movement or appearance, which is leijailee (floats) here. Just like a standard existential sentence, it starts with the location (pihalle), then the verb, and finally introduces what is appearing there as new information.

Why is the subject in the partitive case (aavemaista usvaa) instead of the nominative (aavemainen usva)?

In an existential sentence, the subject takes the partitive case if it represents an indefinite amount of an uncountable mass noun, such as mist, water, or snow. This signals that some ghostly mist is floating into the yard, rather than a specific, previously mentioned mist.

Why use the allative case pihalle (onto the yard) instead of a static location like pihalla (in the yard)?

Dynamic existential clauses often focus on something appearing into a space. Because leijailla (to float or drift) is acting as a verb of motion bringing a new element into the scene, the location takes the directional allative case (-lle) to show the mist is arriving onto the yard.

How would this sentence change if I wanted to say "The ghostly mist floats onto the yard"?