Ich brauche den Reifenheber.

Because Reifenheber is the direct object of brauchen.

• brauchen usually takes the accusative case
• A masculine noun in the accusative singular uses den
• So:
  • nominative: der Reifenheber
  • accusative: den Reifenheber

That is why the sentence says Ich brauche den Reifenheber.

It is in the accusative case.

In this sentence:

• Ich = the subject
• brauche = the verb
• den Reifenheber = the thing being needed, so it is the direct object

In German, direct objects are usually in the accusative.

You usually learn the noun together with its article: der Reifenheber.

There is also a useful compound-noun rule here:

• Reifenheber = Reifen
  • Heber
• In German compound nouns, the last part determines the gender
• der Heber is masculine
• So der Reifenheber is also masculine

That is why the accusative form becomes den Reifenheber.

Because German normally writes compound nouns as one word.

So instead of writing something like tire lever as two words, German combines the parts:

• Reifen = tire
• Heber = lifter/lever

Together: Reifenheber

This is very normal in German. English often uses two words where German uses one compound noun.

The -n is a signal that this masculine noun is in the accusative singular.

Compare:

• der Reifenheber = nominative
• den Reifenheber = accusative

For a native English speaker, this is important because English articles do not change this way, but German articles often do.

Here it is the 1st person singular form:

• ich brauche = I need

A few present-tense forms are:

• ich brauche
• du brauchst
• er/sie/es braucht
• wir brauchen
• ihr braucht
• sie/Sie brauchen

So Ich brauche den Reifenheber uses the correct form for ich.

Usually, no, not in normal standard German if you mean a specific tire lever.

Ich brauche den Reifenheber sounds natural when you mean the tire lever.

If you leave out the article, it sounds incomplete or unusual in this context. German often uses an article where English might sometimes leave one out.

If you meant tire levers in a very general or indefinite sense, you would normally say something else, for example:

• Ich brauche einen Reifenheber. = I need a tire lever.
• Ich brauche den Reifenheber. = I need the tire lever.

Yes. benötigen also means to need.

But there is a small difference in tone:

• Ich brauche den Reifenheber. = more common, everyday
• Ich benötige den Reifenheber. = a bit more formal or stiff in many situations

In ordinary conversation, brauchen is usually the more natural choice.

The sentence Ich brauche den Reifenheber has the normal German main-clause order:

• subject: Ich
• verb: brauche
• object: den Reifenheber

German main clauses usually keep the finite verb in second position.

You can change the order for emphasis:

• Den Reifenheber brauche ich.

That still means the same basic thing, but it puts extra emphasis on den Reifenheber.

A simple pronunciation guide is:

• Ich ≈ ikh or ish, depending on accent and how exact you want to be
• brauche ≈ BROW-khuh
• den ≈ dayn with a shorter vowel than in English day
• Reifenheber ≈ RY-fen-hay-ber

A rough full version:

ikh BROW-khuh dayn RY-fen-hay-ber

A couple of useful notes:

• ich has a German sound that does not exist exactly in English
• eu in brauche sounds like oy
• ei in Reifen sounds like English eye

Because Reifen is part of a compound noun, not a separate word in the sentence.

In den Reifenheber:

• the whole noun is Reifenheber
• only the article shows the case change here: den
• the noun itself stays Reifenheber

So you should think of Reifenheber as one vocabulary item, not as Reifen plus Heber behaving separately in the sentence.

Yes. In standard German, all nouns are capitalized.

So in this sentence:

• Ich is capitalized because it starts the sentence
• Reifenheber is capitalized because it is a noun
• brauche and den are not capitalized because they are not nouns

This is one of the most noticeable spelling differences between English and German.