Agreement in Case

1.  If a sentence contains two verb predicates, both of them governing the same case, the agreement presents no problem:

Он мне мешает и надоедает.

Не disturbs me and annoys me. (dative)

2. When the two verbs of a predicate govern different cases, then two objects (direct or indirect) are used:

Она любила этот портрет (accusative) и дорожила им (instrumental).	She loved this portrait and valued it.
Она не любила её (accusative) и завидовала ей (dative).	She did not like her and envied her.

3. Nouns and adjectives comprising predicatives can be used in either nominative or instrumental case.

   (a)   As for nouns, the choice between the two cases depends on the context: nominative case points out at a steady quality of the object, at its constantly intrinsic characteristic, whereas instrumental case implies a temporary characteristic: 

Андрей был сын (nominative) нашего директора.	Andrew was the son of our boss. (And still is)
Он был лучшим учеником (instrumental) в школе.	He used to be the best student in school. (But no longer is)

However, this differentiation is not so rigid and both cases can be equally used:

Дом, мимо которого она бежала, был городской школой (instrumental).	The house she was running past was a town school.
Мой папа был когда-то инженер (nominative).	My father was an ingeneer some time ago.

   (b)   The instrumental case is normally used with nouns after the following link-verbs:  звать - to call, to name; зваться - to be called; называться - to be called; стать - to become; являться - to be, to appear; казаться - to seem, etc. and with some significant verbs like: жить - to live; родиться - to be born; сидеть - to sit; ходить - to walk, to go; работать - to work, etc. the instrumental case is used:

Моего сына зовут Петей.	My son is called Petya.
Я работаю официантом.	I work as a waiter.

As an exception, nominative is used with geographical names:

Этот город называется Москва. (not "Москвой")

This city is called Moscow.

    (c)   With adjectives either case can be used, regardless of the link-verb. However, instrumental is characteristic of literary speech, and nominative - of conversational:

Она бывает очень вредная. (Also: вредной)	She can be very naughty.
Лето в этом году оказалось не жарким.  (Also: жаркое)	This year the summer turned out to be not so hot.