Le billet aller-retour indique le siège.

Because French usually needs an article before a noun.

• le billet = the ticket
• le siège = the seat

In a sentence like this, French does not normally say just billet or siège by themselves. Even when English might be more flexible, French generally keeps the article.

Here, le billet is the subject of the sentence, and le siège is the direct object.

Aller-retour means round-trip or return.

Literally:

• aller = to go
• retour = return

Together, un billet aller-retour is a round-trip ticket.

A useful comparison:

• un billet aller-retour = a round-trip ticket
• un billet aller simple = a one-way ticket

It is a fixed expression, and in this use it is normally written with a hyphen: aller-retour.

French often hyphenates certain set combinations, especially ones that function together as a single expression. Here, aller-retour acts like one unit describing the type of ticket.

So:

• un billet aller-retour = a return ticket

In French, descriptive words and expressions often come after the noun.

So French says:

• un billet aller-retour

where English says:

• a round-trip ticket

This is a very common difference between English and French word order. Even though English often puts modifiers before the noun, French often places them after it.

Because the subject is singular: Le billet aller-retour.

The main noun is billet, which is singular, so the verb must also be singular:

• le billet indique = the ticket indicates

If the subject were plural, the verb would change:

• Les billets aller-retour indiquent le siège.

So indique is the third-person singular present form of indiquer.

Because indiquer takes a direct object.

In this sentence:

• Le billet = subject
• indique = verb
• le siège = direct object

So French says:

• indiquer quelque chose = to indicate something

Not:

• indiquer à/de le siège

This is just the normal pattern of the verb indiquer.

No. Siège has more than one meaning.

Common meanings include:

• seat — in a bus, train, plane, stadium, etc.
• headquarters — for a company or organization
• sometimes seat in the abstract sense, like a seat of power

In this sentence, because it is about a ticket, siège clearly means seat.

Yes, it is understandable and grammatical. But in real-life travel French, people might more often say something a little more specific, such as:

• Le billet aller-retour indique le numéro du siège.
• Le numéro du siège est indiqué sur le billet.
• Votre siège est indiqué sur le billet.

Why? Because a ticket usually indicates the seat number, not just the seat in a very general sense.

So the original sentence is fine, but a native speaker might prefer a slightly fuller version depending on context.

Sometimes, yes, but the meaning shifts a little.

• siège = the actual physical seat
• place = a place/spot, often meaning an assigned place

For example:

• Votre place est indiquée sur le billet.
• Le numéro de siège est indiqué sur le billet.

In travel contexts, both can appear, but siège is more concrete if you are talking about the actual seat.

A careful pronunciation guide:

• Le ≈ luh
• billet ≈ bee-yay
• aller-retour ≈ ah-lay ruh-toor
• indique ≈ ahn-deek
• le siège ≈ luh syezh

A smoother approximation of the whole sentence:

luh bee-yay ah-lay ruh-toor ahn-deek luh syezh

A couple of useful notes:

• billet does not sound like English bill-et; it sounds more like bee-yay
• siège has a sound close to syezh
• the final -e in indique is not pronounced

In this expression, aller-retour usually stays the same.

Examples:

• un billet aller-retour
• des billets aller-retour

It does not usually change to match gender or number the way many adjectives do. You can think of it as a fixed travel expression.

So even in the plural, you would normally keep:

• des billets aller-retour