Der Direktflug fliegt morgen ab.

Because the full verb is abfliegen, which means to depart / to take off.

abfliegen is a separable verb:

• ab- = the prefix
• fliegen = the base verb

In a normal main clause, the conjugated part goes to position 2, and the prefix goes to the end:

• Der Direktflug fliegt morgen ab.

You can compare:

• Der Direktflug fliegt ab.
• Wann fliegt der Direktflug ab?

But in the infinitive or in some other forms, it stays together:

• Der Direktflug wird morgen abfliegen.
• ..., weil der Direktflug morgen abfliegt.
• Der Direktflug ist schon abgeflogen.

That is very normal in German. German often uses the present tense to talk about the future, especially when there is a clear time expression like morgen.

So:

• Der Direktflug fliegt morgen ab.

means something like:

• The direct flight departs tomorrow.

German does have a future tense with werden, but it is often unnecessary here:

• Der Direktflug wird morgen abfliegen.

This is grammatical, but the present tense version is usually more natural for schedules and planned events.

Direktflug means direct flight.

It is a compound noun:

• direkt = direct
• Flug = flight

German very often combines words into one noun, so instead of saying direkter Flug here, German normally uses the compound noun Direktflug.

So:

• der Direktflug = the direct flight

A common detail: in everyday use, Direktflug often means there is no change of plane, but it does not always guarantee absolutely no stop. If you want to stress non-stop, German can also use Nonstopflug.

Because Flug is a masculine noun in German, so its nominative singular article is der.

• der Flug
• der Direktflug

In this sentence, der Direktflug is the subject, so it is in the nominative case.

That is why you get:

• Der Direktflug fliegt morgen ab.

German main clauses usually follow the verb-second rule: the conjugated verb comes in the second position.

Here the sentence is structured like this:

• Der Direktflug = position 1
• fliegt = position 2
• morgen = later in the sentence
• ab = final prefix of the separable verb

So:

• Der Direktflug fliegt morgen ab.

This is a very normal placement for a time word.

You can also change the word order for emphasis:

• Morgen fliegt der Direktflug ab.

That means the same thing, but now morgen is emphasized a bit more.

Yes, absolutely.

German lets you move elements to the first position for emphasis, as long as the conjugated verb still stays in second position.

So both are correct:

• Der Direktflug fliegt morgen ab.
• Morgen fliegt der Direktflug ab.

The difference is mainly emphasis:

• Der Direktflug fliegt morgen ab. → neutral
• Morgen fliegt der Direktflug ab. → emphasizes tomorrow

Yes, that is normal.

In English, learners sometimes feel that a flight flies sounds a bit odd, but in German this is completely natural. A flight can be the subject of abfliegen.

So these are all possible:

• Der Direktflug fliegt morgen ab.
• Das Flugzeug fliegt morgen ab.
• Der Flug startet morgen.

All are understandable, though they are not identical in nuance:

• abfliegen focuses on departure
• starten focuses more on starting / taking off

Yes, in many contexts you can.

For example:

• Der Direktflug startet morgen.

This is grammatical and natural. But there is a small nuance:

• abfliegen = to depart / take off
• starten = to start / take off

With flights, abfliegen is very common when talking about scheduled departure.
starten is also possible, but it can sound slightly more focused on the actual takeoff.

So in timetable-style language, fliegt morgen ab is very idiomatic.

The dictionary form is abfliegen.

The form fliegt is the 3rd person singular present tense form of fliegen, used here as part of the separable verb abfliegen.

Conjugation:

• ich fliege ab
• du fliegst ab
• er/sie/es fliegt ab

Since der Direktflug is singular, you use:

• Der Direktflug fliegt ab.

Because German often does not need one.

English often uses:

• will depart
• is going to depart

German frequently just uses the present tense plus a time word:

• Der Direktflug fliegt morgen ab.

That already clearly shows future time because of morgen.

So German is often more economical here than English.

Normally, no.

In a full normal sentence, you would usually say:

• Der Direktflug fliegt morgen ab.

German generally needs an article or some other determiner with a singular countable noun:

• der Direktflug
• dieser Direktflug
• ein Direktflug

Leaving it out would sound incomplete in ordinary speech.

You might see article-less language only in headlines, notes, or very telegraphic style, not as standard everyday grammar.

In a subordinate clause, the separable verb is written as one word again, and the verb goes to the end.

For example:

• Ich weiß, dass der Direktflug morgen abfliegt.
• Weil der Direktflug morgen abfliegt, müssen wir früh los.

Notice that in subordinate clauses you get abfliegt, not fliegt ... ab.

That is a very important pattern with separable verbs.