Ich lege den Spitzer in das Federmäppchen.

Because Spitzer is the direct object of the verb legen here, so it must be in the accusative case.

• der Spitzer = nominative (the subject form)
• den Spitzer = accusative (the direct object form)

In this sentence:

• Ich = the subject
• lege = the verb
• den Spitzer = the thing being placed

So German changes der to den because the sharpener is what is being put somewhere.

Because in is a two-way preposition in German. That means it can take either:

• accusative when there is movement toward a destination
• dative when there is location/no movement

Here, the sharpener is being moved into the pencil case, so German uses the accusative:

• in das Federmäppchen = into the pencil case

Compare:

• Ich lege den Spitzer in das Federmäppchen. = I put the sharpener into the pencil case.
• Der Spitzer ist in dem Federmäppchen. = The sharpener is in the pencil case.

Yes. In das is very commonly shortened to ins.

So this sentence can also be:

• Ich lege den Spitzer ins Federmäppchen.

Both are correct. Ins is simply the contracted form of in das.

Because all nouns are capitalized in German.

So in this sentence, these words are capitalized because they are nouns:

• Spitzer
• Federmäppchen

This is a standard rule in German, unlike English, where common nouns are usually lowercase.

Yes, in this sentence lege means put or more specifically lay/place.

The verb is legen, which usually means to put something down in a lying/horizontal position, or more generally to place something somewhere.

German often distinguishes between different kinds of put:

• legen = lay/place something
• stellen = stand/place something upright
• setzen = set/place something in a sitting position

In everyday speech, learners will often see legen used very often for objects like pencils, books, paper, or small items being placed somewhere.

Because in a normal German main clause, the finite verb usually comes in second position.

The sentence structure is:

• Ich = first element
• lege = finite verb in second position
• den Spitzer in das Federmäppchen = the rest

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

For example:

• Ich lege den Spitzer in das Federmäppchen.
• Heute lege ich den Spitzer in das Federmäppchen.

In the second example, Heute is first, so lege still stays second.

• der Spitzer is masculine
• das Federmäppchen is neuter

That is why the articles look like this:

• den Spitzer = masculine accusative
• das Federmäppchen = neuter accusative

For neuter nouns, das stays das in both nominative and accusative, which is why it does not change here.

It is a compound noun, which is very common in German.

Federmäppchen is built from:

• Feder = originally feather, and historically also connected with writing
• Mäppchen = little pouch/case

In modern German, Federmäppchen means pencil case or pen case.

You do not always need to understand every part of a compound perfectly, but it is useful to notice that German often builds long nouns by joining smaller words together.

Because ich is the subject pronoun, and it is in the nominative case.

German pronouns change depending on their role, but here I am the one doing the action:

• ich = I
• mich = me

Since the sentence means I put..., the correct form is ich.

Not if you want the same meaning.

• in das Federmäppchen means into the pencil case
• auf das Federmäppchen would mean onto the pencil case
• neben das Federmäppchen would mean next to the pencil case

So in is the correct preposition because the sharpener is going inside the pencil case.

Both can sometimes translate as put, but legen is more specific and often more natural when talking about placing an object somewhere.

• Ich lege den Spitzer in das Federmäppchen. = more precise, standard, and natural
• Ich tue den Spitzer in das Federmäppchen. = heard in some varieties of spoken German, but less formal and sometimes regional

For learners, legen is the better choice in a sentence like this.

At the beginning of a German word, sp is usually pronounced shp, not like English sp.

So Spitzer sounds approximately like:

• SHPIT-ser

Also:

• the z in German sounds like ts
• so Spitzer has a ts sound in the middle

A rough English-style guide would be SHPIT-tser.

In a school context like this one, der Spitzer usually means pencil sharpener.

Like many nouns, it can have different meanings in other contexts, but for everyday beginner German, if you see Spitzer with school items such as Federmäppchen, it almost certainly means pencil sharpener.