Ich vermute, dass der Flug spät ist.

Dass means that and introduces a subordinate clause.

So the sentence is built like this:

• Ich vermute = I suspect / I assume
• dass der Flug spät ist = that the flight is late

In English, that is often optional:

• I suspect the flight is late
• I suspect that the flight is late

In German, dass is very commonly used in this kind of sentence.

Because dass creates a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

Main clause:

• Ich vermute ...

Subordinate clause:

• dass der Flug spät ist

That final ist is normal German word order after dass.

Compare:

• Main clause: Der Flug ist spät.
• Subordinate clause: ..., dass der Flug spät ist.

Because der Flug is the subject of the subordinate clause.

Inside dass der Flug spät ist, the thing being described as late is the flight, so it is in the nominative case.

• der Flug = nominative, the subject
• den Flug = accusative, a direct object

You would only use den Flug if the flight were the object of a verb, for example:

• Ich sehe den Flug nicht. = I don’t see the flight.

Vermuten means something like:

• to suspect
• to assume
• to suppose

It suggests that the speaker thinks something is probably true, but is not completely certain.

So Ich vermute, dass der Flug spät ist sounds like:

• I suspect the flight is late.
• I assume the flight is late.

It is a bit less firm than stating it as a fact.

Yes, spät often means late in a time sense, but it can also describe something happening later than expected.

Here, der Flug ist spät means the flight is late.

German speakers also often say:

• Der Flug ist verspätet. = The flight is delayed.

Both are possible, but verspätet is often more specific for transport delays.

They are similar, but not exactly the same.

• spät = late
• verspätet = delayed / running late

For a flight, train, or bus, verspätet is often the more precise word because it directly refers to a delay.

Examples:

• Der Flug ist spät. = The flight is late.
• Der Flug ist verspätet. = The flight is delayed.

A learner should understand both, but verspätet is especially common in travel contexts.

Sometimes, yes, but beginners should usually keep it.

English:

• I think the flight is late.
• I think that the flight is late.

German usually prefers:

• Ich vermute, dass der Flug spät ist.

You may sometimes hear a dass-clause omitted in informal speech, but that usually changes the structure and can be harder to learn correctly. For standard learner German, using dass is the safest choice.

Because all nouns are capitalized in German.

So:

• ich = not capitalized here, because it is a pronoun
• Flug = capitalized, because it is a noun

Other examples:

• der Zug
• die Stadt
• das Haus

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

Because vermute is the correctly conjugated form for ich.

The infinitive is:

• vermuten = to suspect / to assume

For ich, it becomes:

• ich vermute

Present tense:

• ich vermute
• du vermutest
• er/sie/es vermutet
• wir vermuten
• ihr vermutet
• sie/Sie vermuten

So vermuten would only be the dictionary form, not the correct form after ich.

Yes. Those are all possible, but they have slightly different shades of meaning.

• Ich denke, dass ... = I think that ...
• Ich glaube, dass ... = I believe / I think that ...
• Ich vermute, dass ... = I suspect / I assume that ...

Vermuten often suggests a guess based on signs or incomplete information.

So if you saw delays on the board but were not sure, Ich vermute, dass der Flug spät ist would fit well.

Yes. A common alternative is:

• Ich vermute: Der Flug ist spät.

Or more naturally:

• Der Flug ist vermutlich spät.
• Vermutlich ist der Flug spät.

Here vermutlich means presumably / probably.

These versions express a similar idea, but Ich vermute, dass ... is a very clear and useful structure for learners.

Here spät is functioning as a predicate adjective, after the verb ist.

In:

• Der Flug ist spät

spät describes der Flug.

It does not take an ending here because predicate adjectives in German do not get adjective endings after verbs like:

• sein = to be
• werden = to become
• bleiben = to remain

Compare:

• der späte Flug = the late flight
• der Flug ist spät = the flight is late

It would be:

• Der Flug ist spät.

This is useful because it shows the word-order difference clearly:

Direct statement:

• Der Flug ist spät.

After dass:

• Ich vermute, dass der Flug spät ist.

So the main change is that in the dass clause, the verb ist moves to the end.