N'oublie pas ton maillot, nous allons à la piscine.

Because N'oublie pas is in the imperative, the form used for commands and instructions.

In French imperatives, you normally do not use the subject pronoun:

• Tu oublies = you forget
• N'oublie pas = don’t forget

So French says the command directly, without tu.

This is a very common point for learners.

The verb is oublier. In the normal tu present form, you get:

• tu oublies

But in the affirmative imperative with most -er verbs, the final -s is dropped:

• Oublie ! = Forget!/Remember!

That is why you see:

• N'oublie pas

A useful note: the -s can come back before y or en for pronunciation reasons:

• Vas-y
• Penses-y

But in your sentence, there is no y or en, so it stays oublie.

Because ne becomes n' before a vowel sound. This is called elision.

Since oublie starts with a vowel sound, French shortens ne:

• ne oublie pas → n'oublie pas

This happens all the time in French:

• je aime → j'aime
• ne est pas → n'est pas

So n'oublie pas is just the normal written form.

French makes a negative imperative by putting ne ... pas around the verb:

• Oublie ! = Forget!/Remember!
• N'oublie pas ! = Don’t forget!

So the pattern is:

• ne + verb + pas

Examples:

• Ne parle pas. = Don’t speak.
• Ne mange pas ça. = Don’t eat that.
• N'oublie pas ton maillot. = Don’t forget your swimsuit.

In spoken French, people often drop ne in casual conversation, but in correct standard writing it stays there.

Because maillot is a masculine noun.

French possessive adjectives must agree with the noun possessed, not with the owner. So:

• ton maillot = your swimsuit
• ta serviette = your towel
• tes lunettes = your glasses

It does not depend on whether the person being spoken to is male or female. Even if you are talking to a girl, you still say:

• ton maillot

because maillot is masculine.

In this sentence, maillot means swimsuit.

But maillot can have a few meanings depending on context. For example, it can also mean a sports jersey, especially in expressions like:

• un maillot de foot = a football/soccer jersey

If you are going to la piscine, then maillot is naturally understood as swimsuit, often short for:

• maillot de bain

So ton maillot here is basically your swimsuit.

Both are possible, but they are different in register.

• nous allons = more standard, a bit more formal or careful
• on va = very common in everyday spoken French

So these both work:

• Nous allons à la piscine.
• On va à la piscine.

A textbook or written example often uses nous allons, while real conversation often prefers on va.

Because piscine is a feminine noun, and after à you use the appropriate article:

• à la piscine = to the pool / to the swimming pool

Compare:

• au parc = to the park (le parc)
• à la piscine = to the pool (la piscine)
• à l'école = to school (l'école)

So à + la piscine simply means to the swimming pool.

French often uses the present tense to talk about something that is happening now or is about to happen very soon.

So:

• Nous allons à la piscine.

can mean:

• We are going to the pool
• We’re going to the pool
• We’re heading to the pool

It sounds natural because the action is current or imminent.

French can also use the near future with aller + infinitive, but that would mean something slightly different:

• Nous allons aller à la piscine = We are going to go to the pool

That is grammatical, but much heavier and less natural here.

Yes. In everyday speech, a very common version would be:

• Oublie pas ton maillot, on va à la piscine.

What changes?

1. Ne is often dropped in speech:

   • N'oublie pas → Oublie pas

2. Nous is often replaced by on:

   • nous allons → on va

This casual version is extremely common in spoken French, but the original sentence is better for standard written French.

Then you would use the vous imperative and the possessive votre:

• N'oubliez pas votre maillot, nous allons à la piscine.

Changes:

• oublie → oubliez
• ton → votre

Use this when:

• talking to several people
• talking to one person formally

If you were talking to several people informally, you could also say:

• N'oubliez pas vos maillots if each person has their own swimsuit

The exact possessive can vary depending on whether you mean one shared item or separate ones, but votre/vos is the key change for vous.

A common difficulty is that ll here does not sound like English l + l.

maillot is pronounced roughly like:

• my-yo

More precisely in French phonetics, it is /ma.jo/.

So:

• maillot sounds close to mah-yo

This same ill / ll pattern appears in words like:

• fille = girl
• bouteille = bottle

though the exact sound can vary depending on the spelling pattern and the word.