Wenn meine Schwester heute Abend anruft, erzähle ich ihr, dass der Kuchen im Backofen fast fertig ist.

Because wenn meine Schwester heute Abend anruft is a subordinate clause introduced by wenn. In German, the conjugated verb usually goes to the end of a subordinate clause.

So:

• wenn = if / when
• meine Schwester heute Abend anruft = my sister calls this evening

This is a very common pattern:

• Wenn er kommt, ...
• Dass sie müde ist, ...
• Weil ich keine Zeit habe, ...

In all of these, the finite verb goes to the end of the subordinate clause.

Because the sentence begins with a subordinate clause: Wenn meine Schwester heute Abend anruft. In German, the verb in the main clause must still be in second position.

The whole wenn-clause counts as position 1, so the verb erzähle has to come next:

• Wenn meine Schwester heute Abend anruft, erzähle ich ihr ...

Structure:

1. Wenn meine Schwester heute Abend anruft
2. erzähle
3. ich
4. ihr
5. rest of the sentence

This is called the verb-second rule in main clauses.

Yes, that is grammatically possible, but it sounds less natural in many contexts because the wenn-clause at the end may briefly sound like it belongs only to the second part.

The more natural version is often:

• Wenn meine Schwester heute Abend anruft, erzähle ich ihr, dass ...

You can also say:

• Ich erzähle ihr, dass der Kuchen im Backofen fast fertig ist, wenn meine Schwester heute Abend anruft.

But learners should be careful, because word order and meaning can feel a little less clear. Starting with the wenn-clause is very standard and natural.

Because erzählen usually takes a dative person and an optional thing being told.

Here:

• ich erzähle ihr ... = I tell her ...
• ihr is the dative form of sie

Forms of sie:

• nominative: sie = she
• accusative: sie = her
• dative: ihr = to her

German uses the dative here because the idea is tell something to someone.

Compare:

• Ich sehe sie. = I see her.
• Ich erzähle ihr etwas. = I tell her something.

Because German often expresses that meaning through case, not through a separate word like English to.

In English:

• I tell her
• I give the book to her

In German, the dative form already shows that function:

• Ich erzähle ihr ...
• Ich gebe ihr das Buch.

So ihr already means something like to her here.

Because dass also introduces a subordinate clause, and in subordinate clauses the conjugated verb goes to the end.

So:

• dass = that
• der Kuchen im Backofen fast fertig ist = the cake is almost ready in the oven

This follows the same rule as the wenn-clause:

• wenn ... anruft
• dass ... ist

This is one of the biggest word-order patterns in German.

Because der Kuchen is the subject of the dass-clause.

In:

• dass der Kuchen im Backofen fast fertig ist

the cake is the thing that is almost ready, so it must be in the nominative case.

• nominative masculine: der Kuchen
• accusative masculine: den Kuchen

You would use den Kuchen if it were a direct object, for example:

• Ich esse den Kuchen. = I eat the cake.

But here the cake is not being acted on directly; it is the subject of ist.

Im is a contraction of in dem.

• in dem Backofen → im Backofen

German very often contracts certain preposition + article combinations:

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

So im Backofen simply means in the oven.

Because in can take either accusative or dative, depending on whether it describes:

• movement toward somewhere → accusative
• location in a place → dative

Here it describes location:

• der Kuchen ist im Backofen = the cake is in the oven

So German uses the dative.

Compare:

• Ich stelle den Kuchen in den Backofen. = I put the cake into the oven.
  
  • movement → accusative
• Der Kuchen ist im Backofen. = The cake is in the oven.
  
  • location → dative

German often uses time expressions without a preposition where English might use one.

So:

• heute Abend = this evening / tonight
• morgen = tomorrow
• nächste Woche = next week
• letzten Monat = last month

You do not need something like an or in here.

Because German very often uses the present tense to talk about the future when the time is clear from context.

Here, heute Abend already shows future time, so present tense is completely normal:

• Wenn meine Schwester heute Abend anruft ...
• erzähle ich ihr ...

English often does something similar:

• If she calls tonight, I’ll tell her ...

German does not need a special future form here. You could use werden, but it would usually sound unnecessary:

• Wenn meine Schwester heute Abend anrufen wird ... is not natural here.

Because wenn is used for:

• repeated events in the present or future
• conditions
• general when/if

In this sentence, the idea is future and conditional:

• If/When my sister calls tonight ...

So wenn is correct.

Als is used for a single completed event in the past:

• Als meine Schwester gestern Abend anrief, ... = When my sister called yesterday evening, ...

So:

• future or conditional → wenn
• one-time past event → als

Both can be translated as to tell, but they are not always used in exactly the same way.

• erzählen often means to tell in the sense of relating information, telling a story, explaining something
• sagen often means to say

In this sentence, erzählen sounds like I’ll tell her / let her know. It works well.

You could also hear:

• ... sage ich ihr, dass ...

That would also be possible, though erzählen can sound a little more like passing on information in a fuller way.

Because in German, subordinate clauses are separated by commas.

Here there are two subordinate clauses:

• Wenn meine Schwester heute Abend anruft
• dass der Kuchen im Backofen fast fertig ist

So commas are required:

• Wenn meine Schwester heute Abend anruft, erzähle ich ihr, dass der Kuchen im Backofen fast fertig ist.

German comma rules are stricter than English in this area. With wenn, dass, weil, ob, and many other subordinating conjunctions, the comma is normally mandatory.

Fast fertig means almost ready or almost done.

• fast = almost
• fertig = ready / finished / done

So:

• Der Kuchen ist fast fertig. = The cake is almost ready.

This is a very common expression in everyday German:

• Ich bin fast fertig. = I’m almost done.
• Das Essen ist fast fertig. = The food is almost ready.

Usually, in a sentence like this, dass should be kept.

English can say:

• I tell her that the cake is almost ready.
• I tell her the cake is almost ready.

German normally prefers:

• Ich erzähle ihr, dass der Kuchen fast fertig ist.

Leaving out dass is much less common and can sound awkward or nonstandard in many contexts. For learners, the safest choice is to keep dass.

Backofen specifically means a baking oven or the normal kitchen oven. In everyday German, it is the standard word for the oven you bake in.

People may also say simply Ofen in some contexts, but Backofen is clearer because Ofen can also mean other kinds of ovens or stoves.

So here:

• im Backofen = in the oven

is the most natural and precise choice.