Das Auto überfährt das Fahrrad.

Because both Auto and Fahrrad are neuter nouns in German.

• das Auto = the car
• das Fahrrad = the bicycle

Also, in German, the nominative and accusative forms of the article are the same for neuter nouns:

• nominative: das
• accusative: das

So in this sentence, both articles happen to look identical even though one noun is the subject and the other is the object.

In a normal German main clause, the subject usually comes before the verb, and the object comes later.

So in:

Das Auto überfährt das Fahrrad.

the most natural reading is:

• Das Auto = subject
• überfährt = verb
• das Fahrrad = object

So the sentence means that the car is doing the action to the bicycle.

If German wanted to make the object come first, it could do that too, but then context or emphasis would matter more:

Das Fahrrad überfährt das Auto would mean the bicycle runs over the car, which is grammatically possible but logically unusual.

überfährt is the 3rd person singular present tense form of überfahren.

Here it matches the subject das Auto, which is singular:

• ich überfahre
• du überfährst
• er/sie/es überfährt
• wir überfahren
• ihr überfahrt
• sie/Sie überfahren

So überfährt means runs over or drives over in this sentence.

Because fahren is a strong verb and changes its stem vowel in the 2nd and 3rd person singular present tense.

The pattern is:

• ich fahre
• du fährst
• er/sie/es fährt

Since überfahren is built from fahren, it keeps that same vowel change:

• ich überfahre
• du überfährst
• er/sie/es überfährt

So the a → ä change is completely normal here.

Here it is inseparable.

That matters because German has both:

• überfahren (inseparable) = to run over
• überfahren / über|fahren (separable, different stress) = a different meaning, such as to cross over by vehicle in some contexts

In your sentence, the meaning is clearly the inseparable verb: to run over.

That is why in a main clause it stays together:

• Das Auto überfährt das Fahrrad.

It does not split into something like fährt ... über in this meaning.

This is a very useful question, because the meanings are not the same.

• überfährt = runs over
• fährt über = usually drives over/across something in a more literal directional sense

Compare:

• Das Auto überfährt das Fahrrad.
  = The car runs over the bicycle.

• Das Auto fährt über die Brücke.
  = The car drives over the bridge.

So the one-word verb überfahren in your sentence is not just a random spelling choice; it gives a specific meaning.

Yes. The stress changes.

• Inseparable überfahren (to run over) has the stress on fahren
• Separable über|fahren often has the stress on über

This stress difference helps native speakers distinguish the meanings.

So in your sentence, the run over meaning uses the inseparable pattern.

Because many German nouns do not change form in the singular when the case changes. Instead, German often shows case through:

• the article
• the word order
• the context

Here:

• das Auto could be nominative or accusative
• das Fahrrad could also be nominative or accusative

So you rely mainly on normal sentence structure: subject first, then verb, then object.

Yes. That would be perfectly grammatical.

• Das Auto überfährt das Fahrrad. = The car runs over the bicycle.
• Ein Auto überfährt ein Fahrrad. = A car runs over a bicycle.

The difference is just definiteness:

• das = the
• ein = a/an

Because German main clauses normally follow the V2 rule: the finite verb goes in the second position.

In your sentence:

Das Auto | überfährt | das Fahrrad

• position 1: Das Auto
• position 2: überfährt

This is one of the most important patterns in German word order.

Yes, but the verb still has to stay in second position in a main clause.

For example:

Das Fahrrad überfährt das Auto.
This changes the meaning, because now the bicycle is the subject.

You could also front another element in a different kind of sentence, but with this exact sentence and these forms, the normal order is the clearest.

Because both noun phrases look the same in nominative and accusative, changing the order can make interpretation harder unless context makes it obvious.

Yes. überfährt is present tense.

So literally it is:

The car runs over the bicycle / The car is running over the bicycle, depending on context.

German present tense often covers both simple present and present progressive meanings that English distinguishes.

A common past form would be the perfect tense:

Das Auto hat das Fahrrad überfahren.
= The car ran over the bicycle.

You may also see the simple past, though it is less common in everyday speech for many verbs:

Das Auto überfuhr das Fahrrad.

The perfect tense is usually the most useful form for learners to know first.

Yes, Auto is neuter: das Auto.

German grammatical gender does not always match what an English speaker might expect. You simply have to learn each noun together with its article:

• das Auto
• das Fahrrad

That is one reason German learners are often encouraged to memorize nouns with the article, not by themselves.

Usually überfahren strongly suggests running over something or someone, often in an accident-like sense.

If you just wanted to describe movement over something more neutrally, German would often use a different structure, such as fährt über with a noun that makes sense for crossing, for example a bridge.

So Das Auto überfährt das Fahrrad strongly points to the idea that the car runs over the bicycle.

The pattern is:

Subject + verb + direct object

More specifically:

• Das Auto = subject
• überfährt = finite verb
• das Fahrrad = direct object

So this is a very standard transitive sentence in German.