Die Lehrerin erklärt den Imperativ Schritt für Schritt.

Lehrerin is the feminine form of Lehrer.

• der Lehrer = male teacher
• die Lehrerin = female teacher

German often forms feminine job titles by adding -in to the masculine form.

The sentence uses the definite article die, so Die Lehrerin means the teacher.

German often uses articles where English might also use one, especially with normal countable nouns:

• die Lehrerin = the teacher

Without the article, Lehrerin would sound less complete in a normal sentence like this unless the style or context clearly allowed it.

It is in the nominative case because it is the subject of the sentence — the person doing the action.

Here, the action is explaining, and the teacher is the one doing that:

• Die Lehrerin = subject
• erklärt = verb
• den Imperativ = direct object

Because Imperativ is the direct object of erklärt, so it must be in the accusative case.

Imperativ is a masculine noun:

• nominative: der Imperativ
• accusative: den Imperativ

So:

• Der Imperativ ist wichtig. = The imperative is important.
• Die Lehrerin erklärt den Imperativ. = The teacher explains the imperative.

You know from its dictionary form: der Imperativ.

In German, every noun has a grammatical gender:

• der = masculine
• die = feminine
• das = neuter

Since the basic form is der Imperativ, the accusative form becomes den Imperativ.

Because it agrees with a third-person singular subject: die Lehrerin.

The verb is erklären = to explain.
In the present tense:

• ich erkläre
• du erklärst
• er/sie/es erklärt
• wir erklären
• ihr erklärt
• sie/Sie erklären

Since die Lehrerin = she, the correct form is erklärt.

German main clauses usually follow the verb-second rule.

In this sentence:

• Die Lehrerin = first element
• erklärt = second element

That is the normal pattern in German statements.

Even if you move another element to the front, the verb still stays in second position:

• Schritt für Schritt erklärt die Lehrerin den Imperativ.

Schritt für Schritt is an adverbial expression meaning step by step. It tells you how the teacher explains the imperative.

So the sentence structure is roughly:

• Die Lehrerin = subject
• erklärt = verb
• den Imperativ = object
• Schritt für Schritt = manner/how

It works like an adverbial phrase in English:

• She explains it step by step.

Because this is a fixed idiomatic expression in German.

You do not normally say:

• den Schritt für den Schritt in this meaning

Instead, German uses the set phrase:

• Schritt für Schritt = step by step

It behaves as one expression, much like certain fixed phrases in English.

Because they are nouns, and all nouns are capitalized in German.

So in this sentence:

• Lehrerin = noun
• Imperativ = noun
• Schritt = noun

This is one of the most noticeable spelling rules in German.

Here, die is the definite article meaning the.

It could only mean she/they in other grammatical contexts, but not here, because it comes directly before the noun Lehrerin:

• die Lehrerin = the teacher

So in this sentence, it is clearly an article.

Yes, German word order is flexible, but the finite verb in a main clause normally stays in second position.

For example, these are possible:

• Die Lehrerin erklärt den Imperativ Schritt für Schritt.
• Schritt für Schritt erklärt die Lehrerin den Imperativ.
• Den Imperativ erklärt die Lehrerin Schritt für Schritt.

The meaning stays similar, but the emphasis changes. The original version is the most neutral and natural.

No, erklären is not separable.

That means you do not split it apart:

• correct: Die Lehrerin erklärt den Imperativ.
• not: Die Lehrerin klärt den Imperativ er.

Some German verbs do separate, but erklären does not.