Learn Hardcore German: Der Spieler spricht mit dem Trainer im Stadion. - The player talks with the coach in the stadium.

Der Spieler is the subject → nominative case → masculine nominative = der.
Mit dem Trainer: the preposition mit always takes the dative case → masculine dative = dem.

So:

Masculine nominative: der Spieler (the player = subject)
Masculine dative: dem Trainer (to/with the coach = object of the preposition)

That’s why you get der for Spieler but dem for Trainer in the same sentence.

How do I know that mit takes the dative case?

Some German prepositions are “fixed” to a certain case, and you simply have to learn them.

Mit is one of the classic “always dative” prepositions. A common mnemonic is:

aus, bei, mit, nach, seit, von, zu → always dative.

So you always say:

mit dem Trainer, mit der Freundin, mit den Kindern, etc.

Never mit den Trainer (accusative) or mit der Trainer (wrong form); it must be dative after mit.

What exactly is going on in im Stadion? Why not in dem Stadion?

Im is just the contracted form of in dem:

in + dem = im

German often contracts preposition + definite article for masculine and neuter dative:

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

Im Stadion and in dem Stadion are grammatically equivalent, but im is by far more common in normal speech and writing. You usually only see in dem Stadion if someone wants to put special emphasis on dem (e.g. contrast: in dem Stadion, nicht in der Halle).

Why is it spricht and not sprechen or sprichtet or something like that?

Sprechen is the infinitive (“to speak”). In the sentence we need 3rd person singular (he/she/it).

The verb sprechen is irregular and changes its vowel in the 2nd and 3rd person singular:

ich spreche
du sprichst
er/sie/es spricht
wir sprechen
ihr sprecht
sie/Sie sprechen

So for der Spieler (he), the correct form is spricht.

Could I also say Der Spieler redet mit dem Trainer or Der Spieler sagt…? What’s the difference between sprechen, reden, and sagen?

They’re related but not interchangeable:

sprechen (mit) = to speak (with), to talk (with)
Neutral, a bit more formal than reden; common in standard German.
- Der Spieler spricht mit dem Trainer.
reden (mit) = to talk (with)
More conversational/colloquial in many contexts.
- Der Spieler redet mit dem Trainer. (completely fine)
sagen = to say (something)
You normally use it with what is said, not with mit:
- Der Spieler sagt etwas zum Trainer. (The player says something to the coach.)
- Der Spieler sagt, dass er müde ist.

For the original sentence (just “is talking with”), sprechen or reden are natural. Sagen would usually need direct or indirect speech.

Can I say Der Spieler spricht zu dem Trainer instead of mit dem Trainer?

You can, but the meaning changes slightly.

mit dem Trainer sprechen ≈ “to speak with the coach”
→ implies a two-way conversation; both are talking.
zu dem Trainer sprechen ≈ “to speak to the coach”
→ focuses on one person speaking towards the other; more one-directional.

In many everyday situations, Germans strongly prefer mit when you mean “have a talk / conversation with”:

Most natural here: Der Spieler spricht mit dem Trainer im Stadion.
Der Spieler spricht zum Trainer sounds more like he is addressing the coach (for example, making a remark or giving feedback), not necessarily a full back-and-forth conversation.

Can I move im Stadion earlier in the sentence, like Der Spieler spricht im Stadion mit dem Trainer? Does that change the meaning?

Yes, that’s absolutely correct German, and the basic meaning stays the same.

Both:

Der Spieler spricht mit dem Trainer im Stadion.
Der Spieler spricht im Stadion mit dem Trainer.

mean: The player is talking with the coach in the stadium.

The difference is only in emphasis and flow. German word order after the verb is fairly flexible:

Putting im Stadion earlier can put slightly more focus on the place.
Putting mit dem Trainer first can put more focus on who he’s talking with.

But there is no big grammatical or meaning change here; both orders are natural.

Why is Stadion used with in and not with something like auf?

German distinguishes roughly between:

in = “inside a 3D space / building / enclosure”
auf = “on top of a surface / open area”

A Stadion is viewed as a space or structure you go into, so you say:

im Stadion (in the stadium)
ins Stadion gehen (go into the stadium)

In contrast:

auf dem Platz (on the field/pitch)
auf der Wiese (on the meadow)

So im Stadion is the natural choice for “in the stadium” because it’s considered an enclosed place, not a flat surface.

How would this sentence look in the plural, e.g. “The players are speaking with the coaches in the stadium”?

You need to change the noun and verb forms, and also the article for the plural dative.

Singular:

Der Spieler spricht mit dem Trainer im Stadion.

Plural version:

Die Spieler sprechen mit den Trainern im Stadion.

Changes:

Subject: der Spieler → die Spieler (same noun form, article changes)
Verb: spricht → sprechen (3rd person plural)
Dative plural: dem Trainer → den Trainern
(note the -n at the end of Trainern, which is required for most masculine and neuter nouns in the dative plural)

How do I say this if the player and/or the coach are female?

German has feminine forms of many profession/role nouns.

Singular feminine:

die Spielerin = the (female) player
die Trainerin = the (female) coach

Examples:

Die Spielerin spricht mit dem Trainer im Stadion.
(female player, male coach)
Der Spieler spricht mit der Trainerin im Stadion.
(male player, female coach)
Die Spielerin spricht mit der Trainerin im Stadion.
(female player, female coach)

Notice the article changes for feminine:

Nominative singular feminine: die Spielerin, die Trainerin
Dative singular feminine: mit der Trainerin

Why is it im Stadion (dative) and not ins Stadion (accusative)? How does in work with cases?

The preposition in can take dative or accusative, depending on the meaning:

Dative = location (where something is) → Wo?
Accusative = direction/motion (where something is going/moving to) → Wohin?

Your sentence describes a location:

Der Spieler spricht mit dem Trainer im Stadion.
→ Where is he speaking? In the stadium. (dative → im)

If you describe movement into the stadium, you’d use accusative:

Der Spieler geht ins Stadion. (in + das = ins)
→ Where to is he going? Into the stadium. (accusative)

So:
im Stadion = being located in the stadium (dative)
ins Stadion = going into the stadium (accusative)

How would I pronounce the sentence? Anything tricky for English speakers?

A rough pronunciation guide (standard German):

Der → like “dair”, short e.
Spieler → shpeel-er
- sp at the beginning of a word is pronounced “shp”.
- ie = long ee sound.
- Final -er is often like a very light “uh”: -ə.
spricht → shpricht
- Again sp = shp.
- ch here is the ich-sound [ç], not like English “k” or “sh”.
mit → like English “mitt”, short i.
dem → “daym” (short e).
Trainer → roughly “TRAY-ner” (but rolled/uvular r; first syllable stressed).
im → like English “im”.
Stadion → SHTAH-dee-on
- St at the beginning also sounds like “sht”.
- Stress is usually on the first syllable: SHTA-dion.

Full sentence, loosely:
“Dair SHPEE-ler shpricht mitt daym TRAY-ner im SHTA-dee-on.”

How would I negate this: “The player is not speaking with the coach in the stadium”? Where does nicht go?

For this kind of simple sentence, nicht usually comes after the verb and the object phrase(s), but before any final place/time phrase if you want to negate only part of it.

Common, neutral negation:

Der Spieler spricht nicht mit dem Trainer im Stadion.

Here, nicht comes directly before what is being negated (the whole rest of the predicate), and this is how you’d typically say it.

If you want to negate only the location, you could say:

Der Spieler spricht mit dem Trainer, aber nicht im Stadion.
(The player is speaking with the coach, but not in the stadium.)