Le voyageur valide un billet avant de prendre le train direct.

In French, singular countable nouns usually need a determiner, such as le, la, un, une, ce, mon, etc.

So:

• le voyageur = the traveler
• un voyageur = a traveler

You normally cannot say just voyageur by itself in a sentence like this. French requires an article where English sometimes does not.

Yes, valide is the present-tense form of valider, and its basic meaning is to validate.
However, in travel contexts, valider un billet often means:

• to stamp a ticket
• to validate it in a machine
• to activate it for use

So although the literal meaning is validates, the practical meaning is often the traveler validates/stamps the ticket before taking the direct train.

Un billet means a ticket, while le billet means the ticket.

Using un billet presents it as a general or unspecified ticket. If the speaker and listener were talking about one specific known ticket, French would more likely use le billet.

So:

• un billet = a ticket, any ticket, or one not yet identified
• le billet = the ticket, a specific one already known in context

After avant when it is followed by a verb, French normally uses de + infinitive.

So:

• avant de prendre = before taking
• avant de partir = before leaving
• avant de manger = before eating

This is just the standard construction:

• avant de + infinitive

If a full clause follows, French can use avant que + subjunctive:

• avant qu’il parte = before he leaves

But in your sentence, the same subject is implied, so avant de prendre is the natural structure.

Because it follows de in the expression avant de + infinitive.

French often uses the infinitive after prepositions and set expressions:

• avant de partir
• sans manger
• pour aller

So prendre stays in its dictionary form, the infinitive, rather than being conjugated.

It is understood to be the same person as the subject of the main clause: le voyageur.

So the sentence means:

• The traveler validates a ticket before taking the direct train.

French often leaves this unstated when the subject is the same in both parts of the sentence.

If the subject were different, French would usually need a fuller structure, such as:

• Le voyageur valide un billet avant que son ami ne prenne le train.

In French, most adjectives come after the noun, unlike in English.

So:

• un train direct = a direct train
• un billet valable = a valid ticket
• une voiture rouge = a red car

Some common adjectives come before the noun, but direct normally comes after it here.

Here, direct means direct, as in a train that goes to its destination without certain stops or without requiring a change.

So le train direct means:

• the direct train
• the non-stop or more direct service, depending on context

It describes the type of train, not the manner of taking it.

Because they are different words that happen to look the same in this form.

• valide as a verb = validates
  
  • from valider
• valide as an adjective = valid / able-bodied, depending on context

In your sentence, valide is clearly the verb because it comes after the subject Le voyageur and before the object un billet.

It is the present tense: présent de l’indicatif.

The verb valider is conjugated here as:

• je valide
• tu valides
• il / elle / on valide
• nous validons
• vous validez
• ils / elles valident

So Le voyageur valide = The traveler validates / is validating, depending on context. In a simple sentence like this, English usually translates it as a general present: The traveler validates.

French often prefers a more compact structure when the subject stays the same:

• avant de + infinitive

So instead of saying something equivalent to before he takes the train, French uses:

• avant de prendre le train

This is very common and natural. English can do something similar:

• before taking the train

French simply uses that structure more regularly.

A careful pronunciation is approximately:

Luh vwa-ya-zhur va-leed un bee-yay ah-vahn duh prahndr luh trahn dih-rekt

A few helpful points:

• Le sounds like luh
• voyageur has the j sound of measure
• billet sounds roughly like bee-yay
• avant has a nasal vowel; the n is not fully pronounced
• prendre also has a nasal sound
• direct in French is usually pronounced dee-rekt

In connected speech, le voyageur may sound smoothly linked together, and the whole sentence flows without strong stress on every word the way English often does.