Wenn du keine Zeit hast, schick mir einfach eine Sprachnachricht.

Wenn means if or sometimes when, depending on context. In this sentence, it means if:

• Wenn du keine Zeit hast = If you don't have time

German often uses wenn for everyday if-clauses. It introduces a dependent clause.

Because wenn creates a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• du hast keine Zeit = you have no time
  becomes
• wenn du keine Zeit hast = if you have no time

That final hast is a very common word-order feature in German.

In German, subordinate clauses are separated from the main clause by a comma. Since Wenn du keine Zeit hast is a subordinate clause, it must be followed by a comma before the main clause:

• Wenn du keine Zeit hast, schick mir einfach eine Sprachnachricht.

This comma is required in standard German.

Because Zeit is a noun, and German normally uses kein to negate nouns when there is no article like ein.

• eine Zeit = a time
• keine Zeit = no time / not any time

By contrast, nicht is usually used to negate verbs, adjectives, adverbs, or more specific parts of a sentence.

So:

• Ich habe keine Zeit. = I have no time / I don't have time.

That is the natural German expression.

Schick is the imperative form, used for giving a command or instruction to du.

The normal du form is:

• du schickst = you send

But the command form is:

• Schick! = Send!

So:

• Schick mir einfach eine Sprachnachricht. = Just send me a voice message.

For many verbs, the du imperative is formed by taking the verb stem:

• schicken → schick

Mir means to me. It is the dative form of ich.

German distinguishes between:

• mich = me (direct object, accusative)
• mir = to me / for me (indirect object, dative)

In this sentence, the thing being sent is eine Sprachnachricht, and the person receiving it is mir:

• Schick mir eine Sprachnachricht.
• Send me a voice message.

So mir is correct because it marks the recipient.

Here einfach means something like:

• just
• simply

It makes the suggestion sound easy and casual:

• schick mir einfach eine Sprachnachricht = just send me a voice message

Depending on context, einfach can also mean simple, but here it is an adverb meaning just/simply.

In German imperatives, the subject du is usually left out.

So instead of:

• Du schickst mir ... = you send me ...

you get the command:

• Schick mir ... = send me ...

That is the normal way to form commands with du.

Because eine is the indefinite article a, and Sprachnachricht is the noun voice message.

German articles usually come before the noun:

• eine Sprachnachricht = a voice message

Also, Sprachnachricht is a feminine noun, so the nominative/accusative singular indefinite article is eine.

Yes. German very often combines nouns into one longer noun.

• Sprache = language / speech
• Nachricht = message
• Sprachnachricht = voice message / audio message

This is extremely common in German. English often uses separate words where German uses one compound noun.

Because the speaker wants to begin with the condition:

• Wenn du keine Zeit hast = If you don't have time

German allows that subordinate clause to come first. When it does, it is followed by the main clause:

• Wenn du keine Zeit hast, schick mir einfach eine Sprachnachricht.

You could also reverse the order:

• Schick mir einfach eine Sprachnachricht, wenn du keine Zeit hast.

Both are correct, but starting with wenn emphasizes the condition first.

Yes, the overall sentence structure would change a little, but each clause would still follow its own normal rules.

Current version:

• Wenn du keine Zeit hast, schick mir einfach eine Sprachnachricht.

Reversed version:

• Schick mir einfach eine Sprachnachricht, wenn du keine Zeit hast.

In the subordinate clause, the verb still goes to the end:

• wenn du keine Zeit hast

And in the main clause, the imperative still begins with the verb:

• Schick mir ...

It is informal, because it uses du and the du imperative:

• Wenn du keine Zeit hast ...
• schick mir ...

If you wanted the formal version with Sie, you would say:

• Wenn Sie keine Zeit haben, schicken Sie mir einfach eine Sprachnachricht.

So the original sentence is something you would say to a friend, family member, colleague you are on du terms with, and so on.