Ich öffne das Ventil und flicke den Fahrradschlauch.

Because both nouns are direct objects here, so they are in the accusative case.

• das Ventil stays das in the accusative, because das nouns have the same form in nominative and accusative.
• der Fahrradschlauch changes to den Fahrradschlauch in the accusative, because masculine der becomes den.

So the base dictionary forms are:

• das Ventil
• der Fahrradschlauch

In this sentence, both are things the speaker is acting on, so they are direct objects.

Both das Ventil and den Fahrradschlauch are in the accusative because they are the objects of the verbs:

• Ich öffne das Ventil.
• Ich flicke den Fahrradschlauch.

A good question to ask is: What am I opening? → the valve
And: What am I patching/repairing? → the inner tube

Those answers are the direct objects, so German uses the accusative.

The verbs are conjugated for ich:

• öffnen → ich öffne
• flicken → ich flicke

This is the normal present-tense ending for many verbs: -en becomes -e with ich.

Examples:

• ich öffne
• du öffnest
• er/sie/es öffnet

and

• ich flicke
• du flickst
• er/sie/es flickt

So öffne and flicke are just the regular ich forms.

Because the subject is the same in both parts of the sentence: I do both actions.

German often leaves out the repeated subject after und when it is obvious:

• Ich öffne das Ventil und flicke den Fahrradschlauch.

This means the same as:

• Ich öffne das Ventil und ich flicke den Fahrradschlauch.

The second version is grammatically possible, but usually less natural unless you want extra emphasis.

Yes. German very often makes compound nouns by joining words together.

Here:

• Fahrrad = bicycle
• Schlauch = tube / hose

So:

• Fahrradschlauch = bicycle tube / inner tube

This is completely normal in German. English often uses separate words where German uses one combined noun.

Also, the gender of a compound noun usually comes from the last part.
Since der Schlauch is masculine, der Fahrradschlauch is also masculine.

Because all nouns are capitalized in German, not just proper names.

So in this sentence:

• Ich is capitalized because it starts the sentence.
• Ventil is capitalized because it is a noun.
• Fahrradschlauch is capitalized because it is a noun.

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

A learner-friendly approximation is:

• öffne ≈ UHF-nuh, but with a rounded vowel

A few helpful points:

• ö is not the same as English o
• it is a front rounded vowel, similar to sounds found in some British pronunciations of certain vowels, but there is no exact everyday English match
• ffn can feel a little awkward at first, so learners often practice it slowly: öf-ne
• in normal speech, the middle vowel may sound reduced, so it can come out close to ÖFF-nuh

If you can produce e while rounding your lips more, that gets you closer to ö.

flicken usually means to patch or to mend, especially something like a tire tube, clothing, or something with a small damaged area.

So:

• den Fahrradschlauch flicken = to patch the inner tube

reparieren is broader and means to repair in general.

For a bicycle inner tube, flicken is especially natural because you often repair it by putting on a patch.

So both can make sense depending on context, but flicken is more specific and idiomatic here.

Yes, in this context das Ventil normally means the valve you open while dealing with the bicycle tube.

German Ventil is a general word for valve, so the exact kind of valve depends on context.
Because the sentence also mentions den Fahrradschlauch, most learners should understand it as the valve connected with that tube.

Yes, it is completely normal.

German main clauses usually put the finite verb in second position:

• Ich öffne das Ventil.

Then the second clause after und is also a main clause pattern:

• (ich) flicke den Fahrradschlauch.

So the sentence follows standard German word order:

• subject → verb → object
• and then another coordinated clause with und

That makes it a very natural everyday sentence structure.