Mets l’étiquette sur le grand carton avant de le fermer.

Because this is a command. In French, commands use the imperative, and the subject pronoun is usually left out.

• Tu mets = you put / you are putting
• Mets ! = Put!

So Mets l’étiquette... means Put the label...

Here it is the tu form, so the speaker is talking to one person informally.

The verb is mettre = to put.

In the imperative, the tu form of mettre is:

• mets

So:

• Mets l’étiquette = Put the label

This is something learners often notice because some -er verbs lose their final -s in the tu imperative, for example:

• Parle !
• Regarde !

But mettre is not an -er verb, so it keeps the -s:

• Mets !
• Prends !
• Ouvre !

Because la becomes l’ before a vowel sound. This is called elision.

• la étiquette → l’étiquette

French avoids the awkward vowel clash, so this is the normal form.

The full noun is:

• l’étiquette = the label

Since étiquette is feminine, the article is normally la, but before é- it becomes l’.

In French, many adjectives come after the noun, but some common ones often come before it. Grand is one of those.

So:

• un grand carton = a big box / a large cardboard box

This is normal French word order.

A useful pattern to remember is that some very common adjectives of size, beauty, age, and goodness often go before the noun, such as:

• grand
• petit
• beau
• jeune
• vieux
• bon

So le grand carton sounds natural.

Here, carton means a cardboard box or carton.

So:

• le grand carton = the big cardboard box

Depending on context, carton can also mean cardboard as a material, but in this sentence it clearly refers to a container you can close.

The preposition sur usually means on or onto.

So:

• Mets l’étiquette sur le grand carton = Put the label on the big box

It tells you where to place the label.

French uses sur very naturally for putting something on the surface of another thing, like a label on a box.

Avant de means before doing something.

In French, when before is followed by a verb, you usually use:

• avant de + infinitive

So:

• avant de fermer = before closing

In this sentence:

• avant de le fermer = before closing it

This is the normal structure.

Because after avant de, French uses the infinitive.

So you get:

• avant de fermer
• not avant de fermes
• not avant de fermé

The infinitive is the basic dictionary form of the verb:

• fermer = to close

This is the same idea as English before closing or before you close it, but French structures it as avant de + infinitive.

The le is a direct object pronoun meaning it.

It refers back to:

• le grand carton

So:

• avant de le fermer = before closing it

French often replaces a repeated noun with a pronoun, just like English does.

Without the pronoun, you would be repeating the noun:

• avant de fermer le grand carton

That is also grammatically possible, but avant de le fermer is more natural when the box has just been mentioned.

Because le is masculine singular, and carton is masculine:

• le carton

But étiquette is feminine:

• l’étiquette / la étiquette

If the pronoun referred to étiquette, it would be la, not le.

So:

• le = the box / it
• not the label

Yes. Both are possible:

• Mets l’étiquette sur le grand carton avant de le fermer.
• Mets l’étiquette sur le grand carton avant de fermer le grand carton.

But the second version repeats le grand carton, so it sounds heavier and less natural. French usually prefers the pronoun once the noun has already been introduced.

So avant de le fermer is the smoother choice.

Yes. Mets is the tu imperative, so it is used for:

• one person
• informal situations

If you wanted to be formal, or if you were speaking to more than one person, you would use mettez:

• Mettez l’étiquette sur le grand carton avant de le fermer.

So:

• Mets = informal singular
• Mettez = formal singular or plural

A learner may wonder about the sound because the spelling looks tricky.

Roughly:

• Mets sounds like may
• l’étiquette sounds roughly like lay-tee-ket

So the beginning is approximately:

• may lay-tee-ket

Also, the s in mets is normally not pronounced as an s sound. The word sounds like met / may depending on accent and approximation.

The apostrophe in l’étiquette does not create a pause; it flows straight into the noun.

Because after avant, French needs de before an infinitive.

So the correct pattern is:

• avant de + infinitive

Examples:

• avant de partir = before leaving
• avant de manger = before eating
• avant de le fermer = before closing it

So avant le fermer is not correct standard French.