Говори так, чтобы все понимали.

Говори is the imperative form of говорить — it means speak.

More specifically:

• говорить = to speak / to talk
• говори = speak! (addressed to one person, informal)

So this sentence is telling someone what to do.

Compare:

• Говори — speak! (to one person, informal)
• Говорите — speak! (to several people, or to one person formally)

Russian often leaves out the pronoun ты (you), because the verb form already shows it.

Because говорить and сказать are not used in exactly the same way.

• говорить focuses on the process / manner of speaking
• сказать usually focuses on saying something once, or on the completed act of saying it

In Говори так, чтобы все понимали, the idea is:

• Speak in such a way...
• Use a way of speaking that everyone can understand

That is about how you speak, not just one finished statement. So говори is the natural choice.

If you used скажи, it would sound more like:

• Say it in such a way that everyone understands

That is possible in some contexts, but it is a bit different in meaning.

Here так means something like:

• so
• like this / in this way
• in such a way

So Говори так means:

• Speak like this
• Speak in such a way
• Speak so...

In this sentence, так connects naturally with чтобы:

• Говори так, чтобы...
• Speak in such a way that...
• Speak so that...

So так is pointing to the manner of speaking.

Чтобы here means so that.

It introduces a subordinate clause expressing purpose or intended result:

• Говори так, чтобы все понимали
• Speak so that everyone understands

This is a very common Russian structure:

• делай так, чтобы... = do it so that...
• пиши так, чтобы... = write so that...
• объясни так, чтобы... = explain it so that...

So the overall pattern is:

[main action] + так, чтобы + [desired result]

This is one of the things that often confuses English speakers.

Yes, понимали looks like a past plural form, and historically it is built from the past-tense form. But after чтобы, Russian often uses this form to express something like:

• so that they would understand
• so that they can understand
• so that everyone understands

So in this sentence, понимали does not mean literal past time.

After чтобы, this form is used very often for:

• purpose
• desire
• intention
• imagined or desired result

So:

• чтобы все понимали = so that everyone understands / so that everyone would understand

Do not translate it word-for-word as a simple past tense.

Because after чтобы, Russian normally does not use the ordinary present-tense form.

So:

• все понимают = everyone understands (a normal statement)
• чтобы все понимали = so that everyone understands / would understand (purpose or desired result)

That is why понимают would not fit well here.

The sentence is not simply stating a fact. It is expressing a goal:

• speak in a way that leads to understanding

That is exactly the kind of idea чтобы is used for.

Both are possible in some contexts, but the aspect changes the nuance.

• понимали is imperfective
• поняли is perfective

Here, понимали sounds more natural because the sentence is about general, ongoing comprehensibility:

• speak in a way that people are able to follow you
• speak clearly enough for everyone to understand what you are saying

If you said чтобы все поняли, that would focus more on a single completed result:

• so that everyone got it
• so that everyone understood in the end

That version is also possible, but it feels a bit more like a one-time successful outcome.
Понимали is better for a general instruction about speaking clearly.

Because все here means everyone / all people, so it refers to multiple people.

That is why the verb is plural:

• все понимали = everyone understood / would understand

A useful distinction:

• все = everyone / all people
• всё = everything

Compare:

• Все понимали. = Everyone understood.
• Всё было понятно. = Everything was clear.

So the plural verb tells you that все here means all people, not everything.

Because чтобы introduces a subordinate clause.

Russian normally puts a comma before conjunctions like что, когда, если, потому что, чтобы, etc., when they begin a subordinate clause.

So in:

• Говори так, чтобы все понимали

the comma separates:

1. the main clause: Говори так
2. the subordinate clause: чтобы все понимали

This comma is required.

Yes, you can say:

• Говори, чтобы все понимали.

But так makes the sentence more natural and more precise here.

With так, the meaning is clearly:

• Speak in such a way that everyone understands

Without так, the sentence is still understandable, but it can sound a little less smooth or a little less explicitly focused on manner.

So:

• Говори так, чтобы все понимали = very natural, emphasizes how you speak
• Говори, чтобы все понимали = possible, but less elegant in many contexts

It is informal singular, because of говори.

You would say this to:

• one friend
• one child
• one student you address informally
• one person in a casual setting

If you wanted a formal or plural version, you would say:

• Говорите так, чтобы все понимали.

That can mean either:

• speaking to several people
• speaking politely to one person

A fairly close breakdown is:

• Говори = Speak
• так = so / in such a way
• чтобы = so that
• все = everyone
• понимали = understood / would understand

So the structure is basically:

Speak in such a way so that everyone would understand.

Natural English translations would be:

• Speak so that everyone understands.
• Speak in a way that everyone can understand.
• Speak clearly enough for everyone to understand.

The stress is:

Говори́ так, что́бы все́ понима́ли.

This can help with pronunciation:

• го-во-РИ
• ТАК
• ЧТО-бы
• все́
• по-ни-МА-ли

A rough pronunciation guide:

• gava-RI tak SHTO-by fsye pa-nee-MA-lee

The exact sounds are more subtle than English spelling can show, but the stress pattern is useful to know.

Yes, Russian word order is flexible, though not completely free.

The neutral version is:

• Говори так, чтобы все понимали.

You could also hear variations like:

• Так говори, чтобы все понимали.

That version gives more emphasis to так.

But for a learner, the original sentence is the best standard pattern to remember:

[imperative] + так, чтобы + [result/purpose clause]

It is a very useful construction in Russian.