Сначала допей чай, а потом открывай окно.

They work together to mark the sequence clearly:

• сначала = first / at first
• потом = then / afterwards

So Сначала допей чай, а потом открывай окно is literally:

• First finish your tea, and then open the window.

Russian often uses both words together when giving step-by-step instructions. It sounds natural and makes the order very explicit.

допей is the imperative of допить, which means to finish drinking.

So:

• пить = to drink
• выпить = to drink up / drink completely
• допить = to finish drinking what remains

In this sentence, допей чай means:

• finish your tea
• literally, drink the rest of the tea

The prefix до- often adds the idea of doing something to the end.

This is one of the most important things in the sentence: the verbs use different aspects.

• допей comes from допить — a perfective verb
• открывай comes from открывать — an imperfective verb

Why perfective in допей?

Because the speaker wants a completed result before anything else happens:

• Finish the tea first.

Perfective imperatives often focus on a single completed action.

Why imperfective in открывай?

Открывай can sound a bit softer, more conversational, or like:

• then go ahead and open the window
• then open the window (when the time comes)

It does not necessarily mean repeated action here. In everyday speech, imperfective imperatives can sound less sharp or less abrupt than perfective ones.

So the contrast is natural:

• first, complete the tea-drinking
• then, go ahead and do the next action

Yes. Открой окно is also grammatical and very natural.

The difference is mostly one of nuance:

• открой окно = a more direct, one-time command to open the window
• открывай окно = often a bit softer, more conversational, or go ahead and open it

In this sentence, открывай can feel like:

• not yet — first finish your tea; then you can open the window

So both are possible, but открывай fits especially well if the speaker is controlling the order of actions in a slightly less abrupt way.

Yes, чай is in the accusative singular, because it is the direct object of допей.

But for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: чай
• accusative: чай

That is why there is no visible change.

Both а and и can connect parts of a sentence, but they are not identical.

Here, а helps separate the two stages:

• Сначала допей чай, а потом открывай окно.

It sounds like:

• First do this, and then do that.

In Russian, а often marks a transition, contrast, or shift from one idea to the next. It is very natural in step-by-step instructions.

If you used и, the sentence would still be understandable, but а потом is the more idiomatic choice here.

These are singular informal imperative forms, so the speaker is talking to:

• one person
• using ты

That is why the commands are:

• допей
• открывай

If the speaker were addressing:

• more than one person, or
• one person formally (вы),

the forms would be:

• Сначала допейте чай, а потом открывайте окно.

They come from the dictionary forms:

• допить → допей
• открывать → открывай

допей

This is the imperative of a perfective verb. The form is not always fully predictable from English, so it is often best learned together with the verb.

• допить = to finish drinking
• допей! = finish drinking!

открывай

This comes from the imperfective verb открывать.

• открывать = to open
• открывай! = open! / go ahead and open!

The ending -й / -ай is common in imperative forms.

Not necessarily. By itself, it is a normal command or instruction.

Its tone depends on context, intonation, and the relationship between the speakers.

It could sound like:

• a parent speaking to a child
• a friend telling another friend what to do
• someone giving practical instructions

Because открывай is imperfective, it may sound a little less abrupt than открой. But it is still definitely an instruction.

If you wanted to make it politer, you could add something like:

• Пожалуйста, сначала допей чай, а потом открывай окно.

The comma separates two clauses joined by а:

• Сначала допей чай,
• а потом открывай окно.

Each part has its own imperative verb:

• допей
• открывай

So the comma is required in normal Russian punctuation.