Difference between "открыл" and "открывал"

Question

I am not sure I understand the difference between these two verb forms. For example, what is the difference in meaning between:

Кто-то открыл дверь.

and

Кто-то открывал дверь.

As far as I understand the first denotes a concrete action and the second a habit, right?

Answer 1

Not exactly. These are two verbs of the different aspect.

открыть is a perfective form, it denotes a finished action, an action that has reached its result.

— Хорошо бы проветрить.
— Я только что открыл окно.

— It would be nice to have some fresh air.
— I have just opened the window.

открывать is an imperfective form, it denotes an action that is taking place at the moment of speech (a) or at some other moment denoted in the text (b). It may also have other meanings such as a habit (c), a regularly repeating action (d), an ability or a possible action (e), or a "general fact" (f) (Russian linguists use term "общефактическое значение"). The meaning of a "general fact" is different from the meaning of the normal completed action in that it stresses the action. If you want to stress the result, use a perfective form.

(a)
— Хорошо бы проветрить.
— Я как раз открываю окно.

— It would be nice to have some fresh air.
— I am opening the window right now.

(b)
Когда она вошла в комнату, я открывал окно.
when she entered the room, I was opening the window.

(c)
По утрам он первым делом открывал окно в спальне.
Every morning the first thing he did was to open the window.

(d)
Я несколько раз открывал окно и выглядывал на улицу, но ничего необычного не заметил.
I opened the window and peeked outside several times, but I didn't notice anything unusual.

(e)
Наш кот слишком умный, он сам открывает холодильник.
Our cat is way too smart, it can open a fridge on his own.

(f)
Кто-нибудь открывал окно в этой комнате? — спросил детектив.
Has anyone opened a window in this room? — asked the detective.

Answer 2

Кто-то открыл дверь.

Somebody opened the door (and the door is still open).

Кто-то открывал дверь.

Somebody had opened the door (maybe several times), but now the door is closed.

The door has been opened at least once during the time period in question.

Answer 3

Открыл denotes a complete action. E.g.

Он открыл дверь и уведел труп в комнате.

He opened the door and saw a corpse in the room.

Открывал denotes the prolonged action. In English it's conveyed by using progressive tenses.

Он наделал много шума, когда открывал дверь. (if we use открыл here, it would mean that he opened the door and made a lot of noise after that).

He made a lot of noise when he was opening the door.