Der Dieb flieht.

Der is the definite article, so here it means the.

In this sentence, der is also showing that Dieb is:

• masculine in grammatical gender
• in the nominative case because it is the subject of the sentence

So der Dieb is the normal nominative form for the thief.

Because Dieb is a masculine noun in German.

German nouns have grammatical gender:

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

So the dictionary form is der Dieb.

This is grammatical gender, not necessarily natural gender. In this case, though, Dieb usually refers to a male thief. If you specifically want a female thief, you would often say die Diebin.

In German, all nouns are capitalized, not just names.

So:

• der Dieb
• das Haus
• die Frau

This is one of the most noticeable differences from English spelling.

Because the infinitive is fliehen, and here it is conjugated for:

• third person
• singular
• present tense

The subject is der Dieb, which is equivalent to he or the thief, so the verb form is:

• ich fliehe
• du fliehst
• er/sie/es flieht
• wir fliehen
• ihr flieht
• sie/Sie fliehen

So Der Dieb flieht uses the er/sie/es form: flieht.

It can mean either one, depending on context.

German present tense often covers both:

• simple present: flees
• present progressive: is fleeing

So Der Dieb flieht could mean:

• The thief flees
• The thief is fleeing

German usually does not need a separate form like English is fleeing unless you want to add extra words for emphasis.

Usually it is intransitive, which means it does not normally take a direct object.

So Der Dieb flieht is a complete sentence by itself.

You can add more information, for example:

• Der Dieb flieht aus dem Haus.
• Der Dieb flieht vor der Polizei.

But you would not normally use fliehen with a direct object the way you use to steal with one.

Because German main clauses normally place the finite verb in second position.

Here the sentence has:

1. Der Dieb = first element
2. flieht = finite verb in second position

This is a basic German statement pattern:

• Der Mann kommt.
• Die Frau lacht.
• Der Dieb flieht.

If you turn it into a yes/no question, the verb usually comes first:

• Flieht der Dieb?

A helpful approximation is:

• Dieb ≈ deep
• flieht ≈ fleet

A few details:

• ie in German is usually a long ee sound
• the h in flieht is silent here
• the final b in Dieb is pronounced like p at the end of the word
• the final t in flieht is clearly pronounced

So the whole sentence sounds roughly like:

deep fleet

but with German pronunciation.

This is because of a common German pronunciation rule called final devoicing.

At the end of a word, certain voiced consonants are pronounced voiceless:

• b sounds like p
• d sounds like t
• g sounds like k

So Dieb is spelled with b, but at the end it is pronounced more like deep.

You still keep the spelling b because related forms show it clearly, for example:

• Diebe = thieves

Yes. That would also be correct, but the meaning changes slightly.

• der Dieb = the thief
• ein Dieb = a thief

So:

• Der Dieb flieht. refers to a specific thief
• Ein Dieb flieht. introduces or mentions an unspecified thief

English works the same way with the versus a.