N’oublie pas de vérifier la pièce jointe avant d’envoyer le courriel.

Because ne becomes n’ before a vowel sound. The verb oublie starts with ou-, so French uses elision:

ne oublie pas → n’oublie pas

This is very common in French:

• n’oublie pas
• j’aime
• l’email
• d’envoyer

Because this is the tu imperative of an -er verb.

For most -er verbs, the tu form of the imperative drops the final -s:

• tu oublies → present tense
• oublie ! → imperative

So:

• Tu oublies la pièce jointe. = You forget the attachment.
• N’oublie pas la pièce jointe. = Don’t forget the attachment.

One famous exception: the -s can come back before y or en for pronunciation:

• Vas-y
• Manges-en

Because in the imperative, French usually leaves out the subject pronoun.

So instead of saying:

• Tu n’oublie pas...
  French says:
• N’oublie pas...

This is the normal way to give commands, instructions, reminders, and advice.

After oublier, French often uses de + infinitive when it means to forget to do something.

So:

• oublier de vérifier = to forget to check
• oublier d’envoyer = to forget to send

This is a standard pattern:

• J’ai oublié de répondre.
• N’oublie pas de fermer la porte.

French does not use à here.

The sentence is built like this:

• N’oublie pas = negative command
• de vérifier = what not to forget to do
• la pièce jointe = the thing to check
• avant d’envoyer le courriel = when to do it, or before what action

So the structure is basically:

Don’t forget + to check + the attachment + before sending the email

French often uses infinitives after expressions like:

• oublier de
• avant de
• essayer de

In this context, vérifier means to check, to make sure, or to look over.

Although English verify can sound more formal or technical, French vérifier is very natural in everyday situations like this.

Here it means something like:

• check that the attachment is there
• make sure it is the right file
• confirm everything is correct

Literally, pièce jointe means something like attached item or joined document.

• pièce = item, document, piece
• jointe = attached/joined

In modern usage, une pièce jointe is the normal French term for an email attachment.

You may also see:

• PJ = abbreviation for pièce jointe

Because pièce is a feminine singular noun, and jointe agrees with it.

So:

• une pièce jointe = feminine singular
• des pièces jointes = feminine plural

This is adjective or participle agreement, which is very common in French.

Compare:

• un document joint
• une pièce jointe

French uses articles more often than English, and here the speaker is referring to a specific attachment and a specific email in the situation.

So:

• la pièce jointe = the attachment
• le courriel = the email

Even when English might sometimes sound natural without stressing the, French generally keeps the article.

Also remember that the article shows gender:

• la pièce jointe because pièce is feminine
• le courriel because courriel is masculine

Because French uses avant de + infinitive when the subject of both actions is the same.

Here, the same person is:

• checking the attachment
• sending the email

So French says:

• avant d’envoyer

Also, de becomes d’ before a vowel:

• de envoyer → d’envoyer

A useful comparison:

• Avant de partir, ferme la fenêtre. = Before leaving, close the window.

If the subject changes, French usually uses avant que + subjunctive instead.

Yes, courriel is a correct and standard French word for email.

A few usage notes:

• In Canada, courriel is extremely common.
• In France, people often also say mail or e-mail in everyday speech.
• courrier électronique is more formal and longer.

So le courriel is perfectly good French, especially in careful or standard language.

Also, do not confuse:

• courriel = email
• courrier = mail/post

You would use the vous imperative:

N’oubliez pas de vérifier la pièce jointe avant d’envoyer le courriel.

So:

• N’oublie pas... = informal singular, to one person you call tu
• N’oubliez pas... = formal singular or plural, to vous

This is one of the most useful changes to know when moving from informal to formal French.