за

very common #17B1

preposition

1.
behind
Гара́ж нахо́дится за до́мом.
The garage is behind the house.
with the instrumental case (location: where?)
2.
for, in favour of
Мы бо́ремся за демокра́тию.
We are fighting for democracy.
with the accusative case
3.
at, by
Гости сиде́ли за столо́м и у́жинали.
The guests were sitting at the table having dinner.
with the instrumental case (location: where?)
4.
for, because of
Он отсиде́л пять лет за ограбле́ние.
He did five years in prison for robbery.
with the accusative case
5.
behind, over (to the other side of)
Мяч улете́л за забо́р.
The ball flew behind the fence.
with the accusative case (direction: where to?)
6.
at (sitting down), to
Гости сня́ли пальто́ и сели за стол.
The guests took off their coats and sat down at the table.
with the accusative case (direction: where to?)
7.
in, within, over (a period), for (a whole period)
За два ме́сяца он вы́учил русский язы́к.
In two months he learned Russian.
with the accusative case; time in which something is done or that is covered
8.
after, following
Почему́ он бежи́т за на́ми?
Why is he running after us?
with the instrumental case
9.
for, per, a
Я купи́л кни́гу за сто рубле́й.
I bought the book for a hundred rubles.
with the accusative case
10.
for
Его при́няли за её му́жа.
He was mistaken for her husband.
with the accusative case; after принять, считать
11.
for, instead of, on behalf of
Она́ рабо́тает за двоих.
She does the work of two people.
with the accusative case
12.
over, during, at
Мы обсуди́ли э́то за у́жином.
We discussed it over dinner.
with the instrumental case
13.
beyond, on the far side of
Что лежи́т за преде́лами наблюда́емой Вселе́нной?
What lies beyond the edge of the observable universe?
with the instrumental case
14.
out of (town), in the country, to the country
Я был вчера́ за го́родом.
I was out of town yesterday.
in the fixed expressions за городом (where?) and за город (where to?)
15.
over, more than
Я ду́маю, ему уже́ за пятьдеся́т.
I think he is already over fifty.
with the accusative case; also of time: за полночь 'past midnight'

Usage info

за то: for the fact
за это: for this
за...До: From...to

is used for:
(i) Thanking, paying, rewarding, punishing for
(ii) Identifying, evaluating as
(iii) Seizing, holding, leading by
(iv) Feelings experienced on behalf of someone else
(v) The meaning ‘in favour of’

Note:
"Что это за" in the meaning "какой" does not affect the case of the dependent noun.

to the place behind/beyond (movement) - за + object in accusative case
seeking advice: за + Noun (Instrumental)

Examples

Вы ходите держась за руки?
Do you walk hand in hand?
Я не могу́ уследи́ть за все́ми измене́ниями, происходящими в сфе́ре иссле́дования СПИДа.
I can't keep track of all the changes taking place in the world of AIDS research.
Прямо за угло́м.
It's just around the corner.
Я в отве́те за э́тот прова́л.
I am responsible for this failure.
За гора́ми тепле́е.
It is warmer over the mountains.
Я должен присма́тривать за кро́ликами.
I must look after the rabbits.
Волна за волной нака́тывались на пляж.
Wave after wave surged upon the beach.
Кажется, волейболи́ста нака́жут за опозда́ние на игру́.
It seems the volleyball player will be punished for being late for the game.

Typical usagePRO

Movement and location
- пое́хать за грани́цу
  go abroad
- жить за грани́цей
  live abroad
- сесть за руль
  get behind the wheel
- быть за рулём
  be behind the wheel; be driving
Going to get / pick up
- сходи́ть за хле́бом
  go get some bread
- зайти за тобо́й
  stop by to pick you up
- ходи́ть за поку́пками
  go shopping
- верну́ться за ключа́ми
  go back for the keys
Support and voting
- Я за!
  I’m for it!
- голосова́ть за рефо́рму
  vote for the reform
- выступа́ть за мир
  advocate peace
- заступа́ться за дру́га
  stick up for a friend