Notes on Nicholas Maltzoff's Essentials of Russian Grammar
As subject of some negative impersonal constructions, particularly with the negative copula or copula-like verbs. The negative copula looks like: нет, не было, не будет. Maltzoff's examples include: У нас нет хлеба; Там не было мебелм; Завтра не будет уроков.
This can even be encountered with animate subjects: Меня не будет дома. When such phrases are semantically interchangeable with a personal construction—Я не был дома—the impersonal construction implies the frustration or non-fulfillment of an expected event at a specific time/place.
The distinction is not relevant for masculine animate nouns, since the accusative has merged with the genitive in these nouns. Otherwise, with non-negated verbs, the accusative can be said to be the default. The genitive is employed in partitive constructions, either in the singular or plural, denoting an indefinite quantity of an object: купить хлеба, воды, папирос (to buy bread, water, cigarettes). Since the partitive is often restricted in the singular to food or consumables, its use in the plural is more frequent. The accusative is only used of such nouns when the entire thing is referred to.
The distinction is overall similar for negated verbs, but here the genitive is the default, and there are some common circumstances where the accusative should be picked. (1) The accusative is common of feminine nouns. (2) The accusative is used of (a) proper nouns, like names of people or places, (b) 'words pertaining to one particular notion' (балет—ballet, серьёзная музыка—serious music), (c) definite nouns, known to the addressee(s), and (d) of objects which would otherwise be part of a string of genitives.
The long form is often nominal, connoting permanence or objectivity, also in the sense of "a good, big, weird one", as opposed to the temporary or relative (для меня, вас) character of the short form. The short form is the only permissible one when used with всё and это.
In the present, the nominative is the only option. In the past and future, however, the instrumental is normal. The instrumental often refers to a temporary fact of someone, and used alongside temporal adverbs like тогда, в то время. The nominative is for a characteristic which is permanent or independent of time. There are many cases where either can be used. The instrumental is obligatory with the infinitive быть.