Ich flicke den Fahrradschlauch.

Because Fahrradschlauch is the direct object of the sentence, so it is in the accusative case.

• der Fahrradschlauch = nominative singular
• den Fahrradschlauch = accusative singular

The verb flicken is something you do to something, so the thing being repaired takes the accusative:

• Ich flicke den Fahrradschlauch.

This happens because Fahrradschlauch is a masculine noun. In the accusative, masculine nouns change der to den.

You learn the noun together with its article:

• der Fahrradschlauch = the bicycle inner tube

In German, every noun has a grammatical gender:

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

There is not always a logical reason, so it is best to memorize the noun and article as a pair:

• der Fahrradschlauch
• not just Fahrradschlauch

flicke is the 1st person singular form of the verb flicken.

The base form is:

• flicken = to mend, patch, repair

In this sentence:

• ich flicke = I mend / I am mending / I patch

German present tense often covers both simple present and present progressive English meanings, depending on context.

Because German main clauses usually follow the verb-second rule.

In Ich flicke den Fahrradschlauch:

1. Ich = first element
2. flicke = second element
3. den Fahrradschlauch = the rest

This is one of the most important German word-order rules. Even if something else comes first, the conjugated verb still stays in second position:

• Heute flicke ich den Fahrradschlauch.

Yes, it is one word, and it is a compound noun, which is very common in German.

It is made of:

• Fahrrad = bicycle
• Schlauch = tube / hose

So:

• Fahrradschlauch = bicycle inner tube

In German, compound nouns are usually written as one word. The last part determines the grammatical gender, so because der Schlauch is masculine, der Fahrradschlauch is also masculine.

A rough English-friendly guide would be:

• Ich ≈ ikh or ish depending on accent
• flicke ≈ FLIK-uh
• den ≈ dayn with a shorter vowel
• Fahrradschlauch ≈ FAHR-raht-shlauch

A few pronunciation notes:

• ch in ich is the soft German ich-Laut, not like English ch in church
• ck in flicke is just a hard k sound
• sch sounds like English sh
• au sounds like ow in cow

Yes, you could. reparieren also means to repair.

But flicken is especially natural when talking about:

• patching something
• mending something with a hole or tear
• fixing something like a tire tube, clothes, or fabric

So for an inner tube, flicken is a very good and idiomatic choice.

It can mean either one. German present tense often covers both meanings.

So:

• Ich flicke den Fahrradschlauch. can mean:
• I patch the bicycle inner tube.
• I am patching the bicycle inner tube.

The exact meaning depends on context. If you want to make the ongoing action especially clear, German often just relies on context rather than using a special continuous tense like English does.

Yes. den Fahrradschlauch means the bicycle inner tube, so it refers to a specific one already understood from context.

German often uses the definite article where English might sometimes use a possessive:

• Ich wasche mir die Hände. = I wash my hands.

But in your sentence, den Fahrradschlauch simply means the bicycle inner tube. If you want to say my bicycle inner tube, you can say:

• Ich flicke meinen Fahrradschlauch.

Here meinen is accusative masculine, matching Fahrradschlauch.

The singular is:

• der Fahrradschlauch

The plural is:

• die Fahrradschläuche

Examples:

• Ich flicke den Fahrradschlauch. = I mend the bicycle inner tube.
• Ich flicke die Fahrradschläuche. = I mend the bicycle inner tubes.

Notice that the plural article is die, and Schlauch changes to Schläuche.

To some extent, yes, but German word order is not completely free.

The normal order is:

• Ich flicke den Fahrradschlauch.

You can also front another element for emphasis, but then the verb still stays second:

• Den Fahrradschlauch flicke ich.

That version emphasizes the bicycle inner tube. It is grammatical, but less neutral than the original sentence.

What you cannot do in a normal main clause is put the verb later like in English:

• not Ich den Fahrradschlauch flicke.

Yes, in normal standard German that would sound incomplete or incorrect here.

German usually needs an article or another determiner before a singular countable noun:

• den Fahrradschlauch
• meinen Fahrradschlauch
• einen Fahrradschlauch

So you would normally say:

• Ich flicke den Fahrradschlauch.
• Ich flicke einen Fahrradschlauch.
• Ich flicke meinen Fahrradschlauch.

Leaving the article out is not standard in this sentence.

Then the meaning becomes less specific.

• Ich flicke den Fahrradschlauch. = I am mending the bicycle inner tube.
• Ich flicke einen Fahrradschlauch. = I am mending a bicycle inner tube.

Both use the accusative masculine form:

• den for the definite article
• einen for the indefinite article

That is because Fahrradschlauch is masculine and is the direct object.