Le billet aller simple indique le siège.

Because both billet and siège are masculine singular nouns in French.

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

In French, nouns have grammatical gender, so you have to learn each noun with its article when possible.

Aller simple is a fixed French expression meaning one-way.

So:

• un billet aller simple = a one-way ticket

If you translated it literally, aller means to go and simple means simple/single, but together they form the standard expression for a one-way ticket. This is just something you learn as a set phrase.

The opposite is:

• un aller-retour = a round trip / return ticket

In French, descriptive information often comes after the noun.

So:

• un billet aller simple
• literally: a ticket one-way
• natural English: a one-way ticket

This is normal in French. English often puts modifiers before the noun, but French often places them after.

In this expression, aller simple works like a fixed label or descriptive phrase, not like a regular adjective that changes form.

So you say:

• un billet aller simple
• des billets aller simple

The phrase aller simple stays the same. You do not usually change it to match gender or number the way regular adjectives often do.

Because the subject is Le billet aller simple, which is singular.

• Le billet aller simple = the one-way ticket
• indique = indicates

So the verb must also be singular:

• Le billet aller simple indique...

If the subject were plural, then you would use indiquent:

• Les billets aller simple indiquent le siège.

Indique comes from the infinitive indiquer = to indicate.

Here it is in the present tense:

• j’indique
• tu indiques
• il/elle/on indique
• nous indiquons
• vous indiquez
• ils/elles indiquent

So in Le billet aller simple indique le siège, the tense is the present: The one-way ticket indicates the seat.

Using le siège suggests a specific seat—the seat assigned or shown on the ticket.

So the sentence implies something like:

• the ticket indicates the seat
• meaning the specific seat number or seat assignment

If you said un siège, it would sound more general, like a seat, not a particular one.

Siège generally means seat.

It can be used for:

• a seat on a train, plane, or bus
• a chair or sitting place in some contexts

In this sentence, because we are talking about a ticket, le siège clearly means the assigned seat.

Be careful: siège can also mean headquarters in other contexts:

• le siège de l’entreprise = the company headquarters

So context matters.

Because billet aller simple is a standard French expression. French often builds noun phrases like this without adding a word like de.

So you simply say:

• un billet aller simple
• un billet aller-retour

You should learn these as complete travel expressions rather than trying to build them word-by-word from English.

Siège is pronounced roughly like syezh.

The è gives an open eh sound, different from the sound of é.

So:

• siège sounds approximately like syezh
• not like see-egg

The accent helps show the vowel sound and is part of the correct spelling.

Yes, montre is possible in some contexts, but indique is more natural here if you mean that the ticket states, shows, or specifies the seat.

Compare:

• indiquer = to indicate, specify, show clearly
• montrer = to show

With official information on a ticket, indiquer often sounds more precise.

It is understandable and grammatically correct, but in real life French speakers might more often say something a little more specific, such as:

• Le billet indique le numéro du siège. = The ticket indicates the seat number.
• Le siège est indiqué sur le billet. = The seat is indicated on the ticket.

Your sentence is still useful for learning because it clearly shows the structure:

• subject: Le billet aller simple
• verb: indique
• object: le siège