ждать

ждать is unique because it can take either the Accusative or the Genitive case depending on the nature of what you are waiting for.

1. Use the Accusative when the object is specific, concrete, or animate.

People and Animals (Animate) -> Accusative. Я жду сестру. - I am waiting for my sister. Мы ждём друга. - We are waiting for a friend.

Note: For masculine animate nouns like друг, the Accusative form looks identical to the Genitive (друга).

Specific Objects: When you are waiting for a particular, definite thing -> Accusative.

Я жду автобус №5. - I am waiting for bus number 5. Он ждёт эту книгу. - He is waiting for this (specific) book.

2. Use the Genitive when the object is abstract, indefinite, or a general concept.

Abstract Concepts: Ideas, feelings, or events that aren't physical objects. Мы ждём перемен. - We are waiting for changes. Ждать у моря погоды. - Idiom: To wait for the weather by the sea / to wait in vain.

Indefinite Objects: When you are waiting for "some" of something or an unspecified item. Я жду письма. - I am waiting for a letter (any letter, or the expected one in a general sense). Ждать ответа. - Waiting for an answer.

In modern spoken Russian, the distinction is becoming a bit blurred. You will often hear native speakers use the Accusative for concrete inanimate objects (like ждать трамвай instead of ждать трамвая), even if they aren't being strictly "specific." However, for abstract nouns, the Genitive remains the standard.