Je mets les bouteilles d’eau dans la glacière avant de partir à la plage.

Mets is the 1st person singular present-tense form of mettre (to put).

• je mets = I put / I am putting
• tu mets = you put
• il/elle met = he/she puts
• nous mettons = we put
• vous mettez = you put
• ils/elles mettent = they put

So Je mets means I put or I’m putting, depending on context.

In French, the simple present often covers both:

• I put
• I am putting

So Je mets les bouteilles d’eau... can mean either:

• I put the water bottles...
• I’m putting the water bottles...

French does not need a separate progressive form like English does in most everyday cases.

Les is the plural definite article, meaning the.

• la bouteille = the bottle
• les bouteilles = the bottles

Because there is more than one bottle, French uses the plural:

• bouteille → singular
• bouteilles → plural

In French, when one noun describes another, de is often used, and before a vowel it becomes d’.

So:

• bouteilles d’eau = water bottles literally bottles of water

You do not usually say de l’eau here, because this is a naming/type relationship, not a partitive idea like some water.

Compare:

• une bouteille d’eau = a bottle of water
• je bois de l’eau = I drink water / some water

D’ is just a shortened form of de before a vowel sound.

• de + eau → d’eau

This is called elision. French often drops the vowel in small words before another vowel:

• de eau ❌
• d’eau ✅

Dans means in / inside, and la glacière means the cooler.

So:

• dans la glacière = in the cooler

French uses dans when something is being placed inside a container or space.

Examples:

• dans le sac = in the bag
• dans la voiture = in the car
• dans la glacière = in the cooler

Une glacière is a cooler or ice chest—a container used to keep food or drinks cold, especially for trips, picnics, or the beach.

It is a feminine noun, which is why it takes la:

• la glacière

After avant when followed by a verb, French uses de + infinitive.

So:

• avant de partir = before leaving / before I leave

This is a very common structure:

• avant de manger = before eating
• avant de dormir = before sleeping
• avant de partir = before leaving

So de is required here.

In everyday French, partir à is commonly used for going to a destination.

• partir à la plage = to leave for the beach / to go to the beach

You may also hear partir pour in some contexts, but partir à la plage is very natural and common.

Also, à la plage is the normal way to say to the beach or at the beach, depending on context.

French usually needs a preposition and an article before a place noun.

• à la plage = to the beach / at the beach

Because plage is a feminine singular noun, à + la stays à la.

Compare:

• à la plage = to the beach
• au parc = to the park (à + le = au)
• à l’école = to school (before a vowel)

It means before leaving for the beach.

The phrase à la plage goes with partir, not with mets. In other words, the person is putting the bottles in the cooler before going to the beach.

So the structure is:

• Je mets les bouteilles d’eau dans la glacière
• avant de partir à la plage

That final part gives the time/context for the action.

It can describe either, depending on context.

It might mean:

• I’m putting the water bottles in the cooler before leaving for the beach.
  right now, in this moment

or

• I put the water bottles in the cooler before leaving for the beach.
  something I generally do

French present tense is flexible like that.

A simple pronunciation guide is:

Zhuh meh lay boo-tay d-oh dahn lah gla-see-air ah-vahn duh par-teer ah lah plazh

A few helpful points:

• Je sounds like zhuh
• mets sounds like meh
• les sounds like lay
• bouteilles sounds roughly like boo-tay
• eau sounds like oh
• dans has a nasal vowel, roughly dahn
• glacière sounds like gla-see-air
• partir sounds like par-teer
• plage sounds like plazh

The word order is very standard in French:

• Je = subject
• mets = verb
• les bouteilles d’eau = direct object
• dans la glacière = where
• avant de partir à la plage = when / before doing what

So the structure is basically:

Subject + verb + object + place + time/context

This is one reason the sentence is a good model for everyday French.