Bevor ich den Bericht abschicke, muss ich die Zahlen noch einmal nachrechnen.

Because bevor is a subordinating conjunction. It introduces a subordinate clause, and in German subordinate clauses, the conjugated verb goes to the end.

So:

• Ich schicke den Bericht ab. = main clause
• bevor ich den Bericht abschicke = subordinate clause

That final verb position is one of the most important word-order patterns in German.

Because the sentence starts with the subordinate clause:

• Bevor ich den Bericht abschicke, ...

That whole clause takes up the first position in the sentence. In a German main clause, the conjugated verb must come second, so the next part begins with:

• muss ich

This is the normal verb-second pattern:

• Bevor ich den Bericht abschicke, muss ich ...

If you put the main clause first, you get the more familiar order:

• Ich muss die Zahlen noch einmal nachrechnen, bevor ich den Bericht abschicke.

Abschicken is a separable-prefix verb:

• ab-
  • schicken

In a normal main clause, the prefix separates:

• Ich schicke den Bericht ab.

But in a subordinate clause, the verb goes to the end and the prefix stays attached:

• ..., bevor ich den Bericht abschicke.

Here abschicke is the 1st person singular present tense form of abschicken.

Because nachrechnen is also a separable-prefix verb:

• nach-
  • rechnen

Its basic meaning here is to check by recalculating.

When a separable verb appears as an infinitive, it is written as one word:

• nachrechnen

Compare:

• Ich rechne die Zahlen nach. = main clause, prefix separated
• Ich muss die Zahlen nachrechnen. = infinitive, prefix attached

Because müssen is a modal verb, and modal verbs are followed by a bare infinitive in German.

So you say:

• Ich muss nachrechnen.

not:

• Ich muss zu nachrechnen. ❌

Other modal verbs work the same way:

• Ich kann kommen.
• Ich will lernen.
• Ich soll warten.

They are both direct objects, so they are in the accusative case.

• der Bericht → den Bericht in the accusative singular
• die Zahlen stays die Zahlen in the accusative plural

So:

• den Bericht abschicken = to send the report
• die Zahlen nachrechnen = to recalculate/check the numbers

A learner often notices that die Zahlen looks the same as nominative plural, and that is completely normal.

Noch einmal means once again, one more time, or simply again.

In this sentence, it suggests that the speaker wants to check the numbers again before sending the report.

It often has a slightly clearer or more deliberate feel than just wieder. Here, noch einmal nachrechnen strongly suggests doing the calculation another time to make sure it is correct.

In everyday speech, you may also hear:

• noch mal
• nochmals

Because all nouns are capitalized in German.

So:

• der Bericht
• die Zahlen

This is a standard spelling rule, not something special about this sentence.

It is in the present tense:

• abschicke
• muss

German often uses the present tense to talk about something that is happening now or will happen in the near future, especially when the time relationship is already clear from the sentence.

So even though the action may happen in the future, the present tense is perfectly natural here.

Rechnen means to calculate or do arithmetic.

Nachrechnen means to calculate again in order to check.

So in this sentence, nachrechnen is more specific. It is not just doing the calculation for the first time; it is checking the numbers by recalculating them.

That is why nachrechnen fits very well in a context like reports, totals, budgets, or data checking.

Yes. A very natural alternative is:

• Ich muss die Zahlen noch einmal nachrechnen, bevor ich den Bericht abschicke.

This has the same basic meaning.

The difference is mostly about emphasis:

• Starting with Bevor ich den Bericht abschicke puts the focus first on the time condition
• Starting with Ich muss die Zahlen ... puts the focus first on what I have to do

Both are correct and common.