Quick answer
A gerund says what else the subject was doing: Чита́я кни́гу, он засну́л — while reading a book, he fell asleep. It never declines and never conjugates.
Two forms only
| Type | Formed from | Example | Means |
|---|---|---|---|
| imperfective | present stem + -я | чита́я, де́лая, говоря́ | while doing |
| perfective | past stem + -в | прочита́в, сде́лав, сказа́в | having done |
Some verbs have irregular or missing gerunds — быть gives бу́дучи, and пить, петь, ждать have no usable imperfective gerund at all. Nobody expects you to force them.
The same-subject rule
The gerund and the main verb must share one subject. If two different people are involved, you need a full clause with когда́ or по́сле того́ как — breaking this rule is a classic mistake in written Russian, made by natives as well.
Where you will actually meet them
Gerunds are everywhere in written and formal Russian, and they survive in speech in a handful of fixed expressions: не́смотря на (despite), су́дя по (judging by), говоря́ о (speaking of), открове́нно говоря́ (frankly speaking).
The comma is compulsory
A gerund phrase is always set off by commas, wherever it stands in the sentence: Он, прочита́в письмо́, ушёл.
Typical mistakes
Чита́я кни́гу, мне ста́ло гру́стно.
Когда́ я чита́л кни́гу, мне ста́ло гру́стно.
The impersonal мне is not the subject doing the reading.
Прочита́я письмо́…
Прочита́в письмо́…
Perfective gerunds take -в.
Check yourself
Which is the perfective gerund of сде́лать?