Imperative

Three short rules pick the suffix from the present-tense stem and stress:

The crucial diagnostic for -и vs -ь is the я-form stress: if you say я говорю (end-stressed), the imperative is -и; if you say я встану (root-stressed), the imperative is -ь. One small wrinkle: a consonant-stem verb whose stem ends in two consonants always takes -и regardless of stress (so помнить → помнят → помни).

Both aspects form an imperative, and the choice carries meaning. The perfective is the default for a one-time, completed request (прочитай книгу, read the book through). The imperfective signals an ongoing process, repeated action, or, very commonly, invites someone to continue doing something they were already doing (читай, читай, keep reading).

A useful pragmatic rule: the imperfective imperative often sounds inviting or welcoming (входите, садитесь: come in, sit down, the host's idiom). The perfective sounds more like a specific instruction or a brisker request. For a detailed treatment of the aspect pairs themselves, see the Aspect chapter.

Negation flips the default aspect. In a negative command the imperfective is standard: не читай эту книгу (don't read that book). A perfective negative imperative usually means a warning, "be careful not to…", not a prohibition.

Russian doesn't have a direct first- or third-person imperative form, but it expresses the same intent with two small auxiliaries:

давай (singular, familiar) and давайте (plural / polite) form the 1pl hortative. Pair them with a perfective 1pl ("let's do X once") or an imperfective infinitive ("let's start doing X"): давай поедем vs давай ездить.
пусть (neutral) and пускай (colloquial) introduce a third-person imperative; they head a clause with a regular 3rd-person verb form, like English "let him…".

A small group of high-frequency verbs has imperatives you can't derive from the rules above. They're worth memorising as a unit; you'll meet them every day.

Two patterns to notice: most monosyllabic verbs whose stem ends in -й (пить → пей, шить → шей) drop the soft sign and take a bare -й. ехать borrows its imperative from a different verb (поезжать stem) entirely: there is no едь. And будь, будьте is one of the most useful imperatives in the language: it heads polite-set expressions like будьте добры (would you be so kind) and будьте здоровы (stay well / bless you).