En décembre, nous mettons une guirlande rouge dans le salon pour Noël.

Because French often uses en with months, seasons, and years to mean in.

• en décembre = in December
• en été = in summer
• en 2025 = in 2025

You do not use an article here, so it is en décembre, not en le décembre.

The phrase En décembre is a time expression placed at the beginning of the sentence. In French, just like in English, a comma is often used after an introductory phrase.

So:

• En décembre, nous mettons... = natural and clear
• Nous mettons... en décembre = also possible

The comma helps separate the time phrase from the rest of the sentence, but in some short sentences it may be omitted.

French often uses the present tense for things that are:

• habitual
• repeated
• generally true
• done as part of a routine

So nous mettons une guirlande rouge... means something like:

• we put up a red garland
• we put a red garland up
• we usually put a red garland up

It is the normal tense for seasonal habits.

In French, subject pronouns are normally required.

So even though mettons is the we form, French still says:

• nous mettons

and not just:

• mettons on its own

English is similar here: we usually say we put, not just put.

Yes. Mettre means to put and it is an irregular verb.

Its present tense forms are:

• je mets
• tu mets
• il / elle / on met
• nous mettons
• vous mettez
• ils / elles mettent

So mettons is simply the correct nous form of mettre.

Because guirlande is a feminine noun.

That means it takes feminine articles:

• une guirlande = a garland
• la guirlande = the garland

If a noun is feminine in French, the article must match it.

In French, most adjectives come after the noun.

So:

• une guirlande rouge = a red garland

This is the normal position for many color adjectives:

• une voiture rouge
• un pull bleu
• des fleurs blanches

A small group of common adjectives often comes before the noun, but colors usually come after.

It actually already agrees correctly with guirlande, but rouge has the same form in the masculine singular and feminine singular.

Compare:

• un pull rouge
• une guirlande rouge

Both use rouge.

But in the plural, it changes:

• des pulls rouges
• des guirlandes rouges

So agreement is still happening; it is just not visible in the singular.

Dans means in or inside, so dans le salon means in the living room.

• dans = in / inside
• le salon = the living room

Also, salon is a masculine noun, which is why it uses le.

In this sentence, salon means living room, not a beauty salon or hair salon.

Both can exist, but they do different jobs.

• pour Noël means for Christmas or for the Christmas season / occasion
• à Noël means at Christmas

In this sentence, pour Noël shows purpose:

• we put up the garland for Christmas

So it answers the question what for?

Holiday names in French often appear without an article after prepositions such as à or pour.

So you commonly get:

• à Noël
• pour Noël

not:

• pour le Noël

That would sound unnatural here.

In French, months and days of the week are normally not capitalized, so:

• décembre
• lundi

That is different from English.

With Noël, you will often see a capital letter because it is the name of the holiday. In modern French, Noël is very commonly written this way.