Learn Hardcore German: Bitte bring die Unterlagen morgen mit.

It’s the separable prefix of the verb mitbringen (to bring along/with). In main clauses and imperatives, the prefix splits off and goes to the end: bring … mit. In a subordinate clause or when the verb is an infinitive/participle, it stays attached:

Main clause: Ich bringe die Unterlagen morgen mit.
Subordinate clause: …, dass ich die Unterlagen morgen mitbringe.
With a modal: Kannst du die Unterlagen morgen mitbringen?
Perfect: Ich habe die Unterlagen mitgebracht. Note: This mit is a verbal particle here, not the preposition mit that takes the dative (like mit mir, mit den Unterlagen).

Why is it bring and not bringst or bringe?

Because it’s the 2nd person singular imperative. German uses the verb stem for the du-imperative:

du-imperative: Bring (… mit)! (optional -e: Bringe is also correct but less common)
Statement (not a command): Du bringst … mit. So bringst would be a normal present-tense statement, not an imperative.

How do I say this formally or to more than one person?

Formal (Sie): Bitte bringen Sie die Unterlagen morgen mit.
Plural informal (ihr): Bitte bringt die Unterlagen morgen mit. The sentence you have is informal singular (du) without saying du explicitly.

Is die Unterlagen singular or plural? What does it usually mean?

It’s plural and typically means “documents/paperwork.” The singular die Unterlage exists but is less common for “a document” and often means something like “pad/base/support.” For a single document, das Dokument is more idiomatic.

Which case is die Unterlagen here, and how would it look in other cases?

Accusative plural, because it’s the direct object of (mit)bringen. Plural forms:

Nominative: die Unterlagen
Accusative: die Unterlagen
Dative: den Unterlagen
Genitive: der Unterlagen Example with the preposition mit (dative): mit den Unterlagen

Can I drop mit and just say: Bitte bring die Unterlagen morgen?

Not really—without mit or a destination, bringen sounds incomplete. Either keep mit (“bring them along [to where we’ll be]”) or add a destination:

Bitte bring die Unterlagen morgen ins Büro.
Bitte bring die Unterlagen morgen mit. (to wherever we’ll meet)

Is the word order fixed? Where else can I put morgen?

You can move morgen for emphasis/focus, but the separable particle mit must stay last:

Bitte bring morgen die Unterlagen mit.
Bitte bring die Unterlagen morgen mit.
Morgen bring bitte die Unterlagen mit. All are fine; choose based on what you want to emphasize.

Can bitte move around? Does it change the politeness?

Yes, bitte is flexible:

Bitte bring die Unterlagen morgen mit.
Bring bitte die Unterlagen morgen mit.
Bring die Unterlagen morgen bitte mit. Moving bitte doesn’t substantially change politeness; it mostly affects rhythm and emphasis. Starting with Bitte often feels slightly more courteous in writing.

Should this end with an exclamation mark?

Either is fine. An exclamation mark is common for a direct imperative (Bitte bring … mit!). A period is standard in polite emails/messages where it reads like a courteous request (Bitte bring … mit.).

Why is morgen lowercase?

Because it’s an adverb meaning “tomorrow.” The noun der Morgen (the morning) is capitalized. If you mean “tomorrow morning,” say morgen früh or morgen Vormittag.

How do I negate this? Where does nicht go?

Don’t bring them at all tomorrow: Bitte bring die Unterlagen morgen nicht mit.
Not tomorrow (but another day): Bitte bring die Unterlagen nicht morgen, sondern am Donnerstag. Placing nicht before mit negates the verb phrase; placing it before morgen negates the time.

What’s the difference between mitbringen and mitnehmen?

mitbringen: bring something along to the speaker’s/recipient’s location (toward me/us).
mitnehmen: take something along with you from here to somewhere else (away from here). So Nimm die Unterlagen morgen mit means “Take the documents with you tomorrow (to wherever you’re going),” not necessarily to me.

How is bringen different from holen?

bringen: deliver something to a destination.
holen: go get something and bring it back. If you say Bitte hol die Unterlagen, you’re asking the person to fetch them (not just to bring already-possessed documents).

How does the verb behave with modals, other tenses, or in subordinate clauses?

With a modal (infinitive at the end, not split): Kannst du die Unterlagen morgen mitbringen?
Future (infinitive): Ich werde die Unterlagen morgen mitbringen.
Perfect (participle): Ich habe die Unterlagen mitgebracht.
Subordinate clause (conjugated at the end, joined): …, dass du die Unterlagen morgen mitbringst. Only when bringen is the finite verb in main-clause position does the prefix split off: bring … mit.

Any good synonyms for Unterlagen?

Dokumente: documents (neutral, common in IT/legal/admin)
Papiere: papers (more informal)
Akten: files/records (often official, a whole case file)
Belege: receipts/vouchers Choose based on context/register.

Do I need a comma after Bitte?

No comma is required in Bitte bring …. You may insert one if bitte is treated as a parenthetical interjection—Bitte, bring die Unterlagen morgen mit.—to mark a pause, but it’s optional. Both versions are correct.