Je vais t’envoyer le document en pièce jointe par courriel ce soir.

Je vais + infinitive is the near future. It is extremely common in everyday French and often sounds a bit more immediate or natural in speech.

So:

• Je vais t’envoyer... = I’m going to send you...
• Je t’enverrai... = I will send you...

Both are correct. In this sentence, je vais t’envoyer feels very natural because the action is planned for later ce soir.

T’ is the shortened form of te, which means to you.

It becomes t’ because the next word, envoyer, starts with a vowel sound. This is called elision:

• te + envoyer → t’envoyer

So t’envoyer means to send to you.

In French, object pronouns usually go before the verb they belong to.

Here, te belongs with envoyer, not with vais. That is why French says:

• Je vais t’envoyer...

not

• Je vais envoyer te...

This is very common with aller + infinitive:

• Je vais te parler
• Je vais lui écrire
• Je vais le faire

Good question. With a full noun, you would say:

• envoyer le document à Marie

But when à Marie is replaced by an object pronoun, French uses te / lui / leur, and the à disappears:

• envoyer le document à toi → te l’envoyer (in normal French, just te)

So:

• Je vais t’envoyer le document
  means
• I’m going to send you the document

The to idea is already built into te.

Le document is the direct object of envoyer. It is the thing being sent.

So in this sentence:

• t’ = to you
• le document = the thing being sent

French often works like this with envoyer:

• envoyer quelque chose à quelqu’un
• send something to someone

Here that pattern becomes:

• t’envoyer le document
• send you the document

En pièce jointe is the standard expression for as an attachment or attached in email-style French.

Literally, une pièce jointe is an attached document/file or an attachment.

In this sentence, en pièce jointe means that the document will be sent as an attachment, not pasted into the email message itself.

It is a fixed expression, so learners should try to remember it as a chunk:

• envoyer un document en pièce jointe

Because en pièce jointe is the usual idiomatic French expression.

French often uses en in set phrases to mean something like in the form of or as:

• en retard = late
• en avance = early
• en PDF = in PDF format
• en pièce jointe = as an attachment

So this is not something you usually translate word-for-word from English. It is just the normal French way to express this idea.

Par courriel is correct and clear. It means by email.

A few usage notes:

• courriel is especially common in Canadian French
• in France, many people also say par e-mail or par mail
• courriel can sound a bit more standard or formal

So par courriel is absolutely good French, but depending on region, another version may be more common in everyday speech.

Because soir is a masculine singular noun that begins with a consonant sound.

French uses:

• ce before masculine singular nouns beginning with a consonant
• cet before masculine singular nouns beginning with a vowel or mute h

So:

• ce soir
• ce document

but:

• cet après-midi
• cet homme

That is why ce soir is correct.

It can mean either, depending on context.

In English, we often distinguish:

• this evening
• tonight

French ce soir can cover both ideas. In this sentence, either English translation could work depending on the exact context and tone.

The word order can sometimes be changed, but the original sentence is very natural.

Here is the structure:

• Je vais = I’m going to
• t’envoyer = send you
• le document = the document
• en pièce jointe = as an attachment
• par courriel = by email
• ce soir = tonight / this evening

French does allow some flexibility, especially with time expressions like ce soir, but not every rearrangement sounds equally natural. The given order is smooth and clear.

Then French would use another object pronoun:

• Je vais t’envoyer le document
  becomes
• Je vais te l’envoyer

Here:

• te = to you
• l’ = it

Notice that both pronouns come before envoyer. This is a very useful pattern:

• Je vais te le donner
• Je vais lui écrire
• Je vais le leur envoyer

It is mostly neutral.

A few details affect the tone:

• t’ shows the speaker is using tu, so the relationship is familiar or informal
• courriel can sound slightly more careful or formal than mail
• the overall sentence is polite and standard

So it is not slangy, but it is also not overly formal. It sounds like normal, correct written French to someone you address as tu.