Als wir gestern im Innenhof standen, erzählte der Guide eine Geschichte über einen mutigen Ritter.

Because "Als wir gestern im Innenhof standen" is a subordinate clause (Nebensatz) introduced by the subordinating conjunction als.

In German:

• In main clauses, the finite verb is in second position:
  Wir standen im Innenhof. – We were standing in the courtyard.
• In subordinate clauses, the finite verb goes to the end:
  Als wir im Innenhof standen, ... – When we were standing in the courtyard, ...

So als pushes the verb standen to the end of its clause.

No, wenn would sound wrong here.

• als is used for:
  • single events or situations in the past
    → Als wir gestern im Innenhof standen, ... = one specific time yesterday.
• wenn is used for:
  • repeated events in the past, present, or future
    → Wenn wir im Innenhof standen, hörten wir immer Musik. (Whenever we were standing in the courtyard, we always heard music.)
  • conditions (if/whenever):
    → Wenn wir im Innenhof stehen, frieren wir. (If/Whenever we stand in the courtyard, we get cold.)

So because this refers to one specific time yesterday, als is correct and wenn is not.

Both are correct, but the chosen word order is required by German verb-second (V2) rules.

The full sentence is:

• Als wir gestern im Innenhof standen, erzählte der Guide eine Geschichte...

After a subordinate clause at the beginning, the finite verb of the main clause must come immediately after the comma:

• ..., erzählte der Guide eine Geschichte ...

The structure is:

1. [Subordinate clause] Als wir gestern im Innenhof standen,
2. [Verb] erzählte
3. [Subject] der Guide
4. [Object] eine Geschichte ...

If you drop the first clause, normal order applies:

• Der Guide erzählte eine Geschichte über einen mutigen Ritter.

Both forms are possible here:

• Präteritum (simple past):
  Der Guide erzählte eine Geschichte.
• Perfekt (present perfect):
  Der Guide hat eine Geschichte erzählt.

In spoken German, people often use the Perfekt for many verbs in the past.
In written German, especially in narratives or stories, the Präteritum is very common.

Because this sentence is narrating a past event (like in a story or report), erzählte (simple past) fits very well stylistically.

im is a contraction of in dem.

• in dem Innenhof → im Innenhof

The preposition in can take:

• Dative (location: in/at a place): in dem Innenhof → im Innenhof
• Accusative (movement into a place): in den Innenhof

Here, they were standing in the courtyard (location), not going into it (movement).
So you need Dative: in dem Innenhof = im Innenhof.

"Eine Geschichte" is the direct object of the verb erzählen (to tell).

• Wer? (Who?) → der Guide (subject → nominative)
• Was? (What?) → eine Geschichte (direct object → accusative)

You can see the accusative in the article:

• Nominative feminine: eine Geschichte
• Accusative feminine: eine Geschichte (same form for feminine)

So it looks the same as nominative, but grammatically it’s accusative because of its function.

Because "einen mutigen Ritter" is in the accusative singular masculine.

Step by step:

• The preposition über (about) in this sense takes the accusative.
• The noun Ritter (knight) is masculine.

• Adjective + noun with indefinite article in masculine accusative:

  • Nominative: ein mutiger Ritter
  • Accusative: einen mutigen Ritter

So:

• über einen mutigen Ritter is correct.
• über ein mutiger Ritter would be wrong (wrong article and adjective ending).

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

1. Dative – location/position (above, over):

   • Die Lampe hängt über dem Tisch. (The lamp hangs above the table.)

2. Accusative – direction (over/across) or topic (about):

   • Wir gehen über die Brücke. (We are going over the bridge.)
   • Er erzählte eine Geschichte über einen mutigen Ritter. (He told a story about a brave knight.)

In the meaning “about (a topic)”, über always takes the accusative, so we get über einen mutigen Ritter.

In German, all nouns are capitalized, no matter whether they come from German or another language.
Guide is a noun here (a person), so it must be written as Guide.

About usage:

• der Guide is used in modern, often informal or tourism-related contexts and is borrowed from English.
• More traditional German words would be:
  • der Führer (can have problematic historical associations)
  • der Fremdenführer (tour guide)
  • der Reiseführer (usually: guidebook; sometimes also the person)

In many tourist contexts today, der Guide is quite common and sounds natural.

The comma separates a subordinate clause from the main clause.

• Subordinate clause: Als wir gestern im Innenhof standen,
• Main clause: erzählte der Guide eine Geschichte über einen mutigen Ritter.

In German, you must use a comma between:

• A Nebensatz (subordinate clause) introduced by words like als, weil, dass, wenn etc.
• And the main clause to which it belongs.

So the comma here is grammatically obligatory, not optional.

Yes, that sentence is grammatically correct, but it sounds more heavy and less natural for everyday speech.

Possible variants include:

• Als wir gestern im Innenhof standen, erzählte der Guide eine Geschichte über einen mutigen Ritter. (most natural)
• Der Guide erzählte eine Geschichte über einen mutigen Ritter, als wir gestern im Innenhof standen.
• Der Guide erzählte, als wir gestern im Innenhof standen, eine Geschichte über einen mutigen Ritter. (correct, but more formal/literary, and slightly clumsy)

German allows some flexibility in word order, but the original version is the clearest and most natural.

You should not say "Als wir im Innenhof gestern standen" – that sounds wrong or at least very unnatural.

Better options:

• Als wir gestern im Innenhof standen, ... (time before place → very natural)
• Als wir im Innenhof standen, erzählte der Guide gestern eine Geschichte ... (emphasizes that the telling was yesterday)

General tendencies:

• In subordinate clauses, adverbs usually follow the pattern time – manner – place when possible.
• So gestern (time) before im Innenhof (place) is very natural: ... gestern im Innenhof ...

The given sentence is already in a very typical and natural order.