Im Wörterheft sieht meine Lehrerin sofort, wenn in einem Wort ein Buchstabe fehlt.

Im is a contraction of in dem.

• in dem Wörterheft → im Wörterheft
• This means in the vocabulary notebook or in the word notebook

German often contracts:

• in dem → im
• an dem → am
• zu dem → zum
• zu der → zur

Here, dem is dative, because in is expressing a location, not movement.

So:

• im Wörterheft = in the notebook

This is because German main clauses follow the verb-second rule.

The finite verb must come in the second position. If something other than the subject comes first, the verb still stays second, and the subject moves after it.

Normal order:

• Meine Lehrerin sieht sofort ...

But here the sentence begins with Im Wörterheft, so the verb must come next:

• Im Wörterheft sieht meine Lehrerin sofort ...

Structure:

• Im Wörterheft = position 1
• sieht = position 2
• meine Lehrerin = comes after the verb

This is very common in German.

Wörterheft is a compound noun:

• Wörter = words
• Heft = notebook / exercise book

So Wörterheft means something like:

• vocabulary notebook
• word notebook
• exercise book for words/spelling/vocabulary

German uses compound nouns all the time, and the last part usually tells you the basic category of the thing:

• Heft = notebook
• so a Wörterheft is a kind of Heft

Here wenn means something like when or whenever.

In this sentence, it introduces a situation:

• wenn in einem Wort ein Buchstabe fehlt
• when/whenever a letter is missing in a word

Wenn can mean:

• if
• when / whenever

The exact meaning depends on context.

A useful comparison:

• wenn = when/if for repeated situations, general situations, or conditions
• als = when for a single event in the past
• wann = when as a question word

Examples:

• Wenn ich Zeit habe, lerne ich Deutsch. = If/When I have time, I study German.
• Als ich klein war, ... = When I was little, ...
• Wann kommst du? = When are you coming?

So in your sentence, wenn works well because it describes a general situation: whenever a letter is missing, the teacher notices.

Because the clause beginning with wenn is a subordinate clause.

In German subordinate clauses, the conjugated verb usually goes to the end.

So:

• wenn in einem Wort ein Buchstabe fehlt

Structure:

• wenn = subordinating conjunction
• in einem Wort = prepositional phrase
• ein Buchstabe = subject
• fehlt = verb at the end

This is one of the most important German word-order rules.

Compare:

• Main clause: Ein Buchstabe fehlt.
• Subordinate clause: ..., wenn ein Buchstabe fehlt.

Because in is a two-way preposition.

Two-way prepositions can take:

• accusative for movement toward somewhere
• dative for location

Here there is no movement. It means in a word, as a location/context where the letter is missing. So German uses dative:

• in einem Wort

Compare:

• Ich schreibe das Wort in das Heft. = I write the word into the notebook. → movement → accusative
• Das Wort steht im Heft. = The word is in the notebook. → location → dative

Same idea here:

• in einem Wort = in a word

Because the verb fehlen works differently from the English idea of to miss.

In German, fehlen usually means to be missing or to be absent.

So:

• ein Buchstabe fehlt = a letter is missing

Here, ein Buchstabe is the grammatical subject, so it is in the nominative case.

That is why it is:

• ein Buchstabe not einen Buchstaben

This can feel a bit backwards to English speakers, but it is very natural in German.

Fehlen usually means:

• to be missing
• to be lacking
• sometimes to be absent

Examples:

• Ein Buchstabe fehlt. = A letter is missing.
• Mir fehlt ein Euro. = I’m missing a euro / I’m short a euro.
• Du fehlst mir. = I miss you.
  Literally, this is more like you are missing to me.

So in your sentence, German is not saying someone misses a letter. It is saying:

• a letter is missing

That is why ein Buchstabe is the subject.

Sofort means immediately or right away, and adverbs in German are often fairly flexible in position.

Here:

• Im Wörterheft sieht meine Lehrerin sofort ...

This placement is natural because sofort modifies the verb sieht:

• she immediately notices/sees

You could see slightly different word orders in German depending on emphasis, but this version is completely normal.

For example:

• Meine Lehrerin sieht im Wörterheft sofort, wenn ...
• Im Wörterheft sieht meine Lehrerin sofort, wenn ...

Both are possible, but the original puts special focus on in the notebook first.

Because all nouns are capitalized in German.

So:

• das Wörterheft
• die Lehrerin
• das Wort
• der Buchstabe

This is a standard rule in German and applies no matter where the noun appears in the sentence.

Yes. A very straightforward version would be:

• Meine Lehrerin sieht im Wörterheft sofort, wenn in einem Wort ein Buchstabe fehlt.

This has the subject first and may feel more familiar to an English speaker.

The original sentence:

• Im Wörterheft sieht meine Lehrerin sofort, wenn in einem Wort ein Buchstabe fehlt.

puts Im Wörterheft first for emphasis or for a more natural flow in context.

Both are grammatical. The difference is mainly about focus and style, not basic meaning.