Learn Hardcore German: Mithilfe der Kollegin teile ich der Leiterin alles mit und reiche den Zettel ein. - With the help of the female colleague I inform the female leader of everything and submit the slip.

Here it’s the preposition mithilfe (“with the help of”) which normally is written as one word and lowercase. It’s capitalized only because it starts the sentence. You can also write the two-word variant mit Hilfe (with the noun Hilfe), which is a bit more formal/neutral. Both are acceptable.

Which case does mithilfe take, and what is der Kollegin here?

mithilfe governs the genitive, so der Kollegin is genitive singular (feminine). Note: feminine genitive singular looks the same as feminine dative singular (der), so you know it’s genitive because of the preposition. A quick check with a masculine noun shows the difference clearly: genitive = mithilfe des Kollegen, dative = mit Hilfe von dem Kollegen.

Why is the verb split as teile … mit?

Because mitteilen is a separable verb. In a main clause, the finite verb is in second position and the separable prefix (mit) moves to the clause-final position: ich teile … mit. In a subordinate clause, the verb goes to the end and the prefix stays attached: …, weil ich ihr alles mitteile. The participle is mitgeteilt.

How do I know this is mitteilen and not teilen … mit (“to share with”)?

With plain teilen, the person you share with must be introduced by the preposition mit: ich teile alles mit der Leiterin. In the sentence, der Leiterin appears without a preceding mit, which signals the verb is mitteilen (“to inform/communicate [something] to [someone]”), not “share.”

Why is der Leiterin dative?

With mitteilen, the recipient is in the dative and the thing you communicate is in the accusative: jemandem (dative) etwas (accusative) mitteilen. So: der Leiterin (dative) and alles (accusative).

Why is alles placed after der Leiterin? Could I say … teile ich alles der Leiterin mit?

Default order (when both objects are full noun phrases) is dative before accusative, so der Leiterin before alles is very natural. With pronouns, order shifts:

If one object is a personal pronoun, it tends to come earlier: Ich teile es ihr mit / Ich teile ihr das mit.
With indefinites like alles, both orders can occur, but der Leiterin alles is the more idiomatic choice here. … alles der Leiterin … is possible but sounds less natural.

What’s the difference between alles and alle?

alles = “everything” (neuter singular, indefinite pronoun): Ich sage dir alles.
alle = “all” (plural determiner/pronoun) for people/things you can count: Alle Kolleginnen kommen. “All of them” = alle (people/things, plural).

Why is there no comma before und?

German normally does not use a comma before und when coordinating main clauses or predicates. A comma can be added in some cases for clarity, but it’s not required here.

Why is ich omitted in the second part (… und reiche …)?

In coordinated clauses, German often omits a repeated subject if it stays the same. You could also say … und ich reiche den Zettel ein, which is equally correct but a bit more explicit.

Why is ein at the very end?

Because einreichen is separable. In a main clause: ich reiche den Zettel ein (finite verb in second position, prefix at the end). The participle is eingereicht; in a subordinate clause: …, dass ich den Zettel einreiche.

What’s the nuance of einreichen vs abgeben vs einreichen?

einreichen: “to submit” (often in administrative/official contexts: forms, applications).
abgeben: “to hand in/turn in” (more physical act of handing something over, e.g., homework at school). In many contexts both are fine, but einreichen sounds a bit more formal/official.

Why den Zettel and not dem Zettel?

den Zettel is accusative singular (masculine), the direct object of einreichen. dem Zettel would be dative and is not correct here.

How would this sentence look in the past?

Perfekt (very common in speech): Mithilfe der Kollegin habe ich der Leiterin alles mitgeteilt und den Zettel eingereicht.
Präteritum (more written style): Mithilfe der Kollegin teilte ich der Leiterin alles mit und reichte den Zettel ein.

Can I start with Ich instead of Mithilfe der Kollegin?

Yes: Ich teile der Leiterin mithilfe der Kollegin alles mit und reiche den Zettel ein. Fronting Mithilfe der Kollegin puts emphasis on the help; starting with Ich is neutral.

Is there anything to know about Kollegin/Leiterin forms?

Yes, -in marks the feminine form: die Kollegin (fem.) vs der Kollege (masc.), die Leiterin (fem.) vs der Leiter (masc.). Colloquial alternatives include die Chefin for a female boss. If you replaced the nouns with pronouns, you’d get, e.g., Ich teile ihr alles mit.

Could I use sagen instead of mitteilen?

Often yes: Ich sage der Leiterin alles. But mitteilen is more formal/neutral and often used for conveying information/notifications. sagen is broader (“to say/tell”). In official contexts, mitteilen fits better.