Im Seminar hält unsere Teamleiterin einen kurzen Vortrag, und ein Sprecher begrüßt alle Zuhörer und Zuhörerinnen.

Im is just a common contraction of in dem:

• in (preposition) + dem (dative article, neuter, singular) → im

German does this a lot with prepositions + the dative article:

• an dem → am
• in dem → im
• zu dem → zum
• bei dem → beim

Grammatically, im Seminar and in dem Seminar are the same; im simply sounds more natural and is what people normally say.

The preposition in can take either dative or accusative:

• dative = location / time (where? when?)
• accusative = direction (where to?)

In this sentence, im Seminar describes the setting or time (roughly during the seminar, in the seminar). It’s a static situation, not movement towards the seminar, so German uses the dative.

• im Seminar → dative (location / time)
• ins Seminar gehen (go to the seminar) → ins = in das, accusative (direction)

German main clauses follow the verb‑second rule:

• The finite verb must be in the second position.
• Position 1 can be many things: the subject, a time expression, a place expression, etc.
• The subject does not have to be first.

Here:

1. Im Seminar → first element
2. hält → finite verb, must be second
3. unsere Teamleiterin → subject
4. einen kurzen Vortrag → object

So Im Seminar hält unsere Teamleiterin … is correct.
Im Seminar unsere Teamleiterin hält … breaks the verb‑second rule and is ungrammatical.

Teamleiterin is a feminine noun (ending -in is a common feminine ending for job titles).

The possessive unser changes its ending according to gender and case:

• Masculine nominative: unser Chef
• Neuter nominative: unser Büro
• Feminine nominative: unsere Teamleiterin
• Plural nominative (all genders): unsere Kollegen

Here, unsere Teamleiterin is the subject of the clause → nominative, feminine singular → ending -e: unsere.

Vortrag is a masculine noun, and here it is the direct object → accusative case.

For masculine nouns:

• Nominative: ein kurzer Vortrag (subject)
• Accusative: einen kurzen Vortrag (direct object)

In the sentence:

• unsere Teamleiterin → subject (nominative)
• hält → verb
• einen kurzen Vortrag → direct object (accusative)

So the article changes to einen, and the adjective takes -en: kurzen.

Pattern (masculine, with ein):

• Nominative: ein kurzer Vortrag
• Accusative: einen kurzen Vortrag
• Dative: einem kurzen Vortrag
• Genitive: eines kurzen Vortrags

Sprecher is grammatically masculine:

• der Sprecher = male speaker
• In traditional usage, der Sprecher can also be used generically (the speaker without specifying gender).

Zuhörer and Zuhörerinnen are:

• Zuhörer = male listeners (or generic plural in older / less inclusive usage)
• Zuhörerinnen = female listeners

So:

• ein Sprecher → one (grammatically masculine) speaker, gender not explicitly specified
• Zuhörer und Zuhörerinnen → explicitly includes both male and female listeners

In modern German, people often try to avoid using the masculine form as a generic for all genders, especially in formal or public contexts. That’s why the sentence spells out Zuhörer and Zuhörerinnen.

This is about gender‑inclusive language.

Traditionally, the masculine plural (Zuhörer) was used to refer to mixed groups (men + women) or when gender was unknown. Today, many speakers consider this not inclusive enough.

Ways to be inclusive include:

• Zuhörer und Zuhörerinnen (explicitly both forms)
• Zuhörerinnen und Zuhörer (often the feminine first)
• Zuhörende (a gender‑neutral participle form, increasingly used)

The sentence uses Zuhörer und Zuhörerinnen to make clear that all listeners, male and female, are being addressed.

alle vs. allen is a case issue.

• alle = nominative or accusative plural
• allen = dative plural

In the clause ein Sprecher begrüßt alle Zuhörer und Zuhörerinnen:

• ein Sprecher → subject (nominative)
• begrüßt → verb
• alle Zuhörer und Zuhörerinnen → direct object (accusative)

So we need the accusative plural, which is alle.
Allen would be used with the dative, for example:

• Er dankt allen Zuhörern und Zuhörerinnen. (He thanks all the listeners.) → dankt
  • dative → allen

In German, all nouns are capitalized, including:

• Regular nouns: Haus, Seminar, Vortrag
• People/agent nouns derived from verbs:
  
  • Zuhörer (from zuhören – to listen)
  • Sprecher (from sprechen – to speak)
  • Lehrer, Fahrer, Leser, etc.

Even though Zuhörer and Zuhörerinnen come from verbs, they function as nouns in the sentence, so they must be capitalized.

Im Seminar can be understood both locally and temporally, depending on context:

• Local: in the seminar (as an event / as a class)
• Temporal: during the seminar

In everyday usage, im Seminar often means something like during the seminar session, not necessarily physically inside a specific room. Because the rest of the sentence is about giving a talk and greeting the audience, the natural reading is during the seminar.

If you specifically wanted during, you could also say:

• Während des Seminars hält unsere Teamleiterin einen kurzen Vortrag.

You have two independent main clauses:

1. Im Seminar hält unsere Teamleiterin einen kurzen Vortrag
2. ein Sprecher begrüßt alle Zuhörer und Zuhörerinnen

They are joined by und.

In modern German spelling rules, the comma here is optional:

• Im Seminar hält unsere Teamleiterin einen kurzen Vortrag, und ein Sprecher begrüßt alle Zuhörer und Zuhörerinnen.
• Im Seminar hält unsere Teamleiterin einen kurzen Vortrag und ein Sprecher begrüßt alle Zuhörer und Zuhörerinnen.

Both versions are correct.

Writers often use the comma when the clauses are a bit longer or they want to make the structure clearer.