Read & compare the examples paying attention to how the period of time is expressed:

1. Я делал отчёт 2 часа.
I was doing the report for 2 hours.   

2. Я сделал отчёт за 2 часа.
I did the report in 2 hours. 

If we want to emphasize the duration of some action, we don’t use a preposition: for 2 hours = 2 часа. And the action is expressed by an imperfective verb here.

When we want to emphasize finishing an action within a period of time, we use the preposition ‘за’: in 2 hours = за 2 часа. In this case, we use the perfective verb.

Here are more examples:

1. Я читал книгу 3 дня.
I was reading the book for 3 days.

2. Я прочитал книгу за 3 дня.
I read a book in 3 days.

1. Я учил английский язык 4 года в университете.
I was learning English for 4 years at university.

2. Я выучил английский язык за 4 года.
I learnt English in 4 years.

Words of period change depending on number like this: 

 NOTE!

This pattern repeats in compound numbers. To choose the correct word for ‘year’ look at the last digit of the number:

• Use ‘час’, 'день', etc., if the number ends in 1 (except 11).
  21 час, 31 день, 51 год

• Use ‘часа’, 'дня', etc., if it ends in 2, 3 or 4 (except 12–14).
  22 часа, 23 дня, 54 года

• Use ‘часов’, 'дней', etc., in all other cases.
  11 часов, 12 месяцев, 78 лет, 100 дней

Listen to the audio to hear the correct answers

Listen to the audio to hear the correct answers