Je mets un glaçon dans l’eau parce qu’il fait chaud.

Mets is the 1st person singular form of the verb mettre in the present tense.

The verb goes like this:

• je mets = I put / I am putting
• tu mets
• il/elle/on met
• nous mettons
• vous mettez
• ils/elles mettent

So because the sentence starts with Je, you need je mets.

Here, mettre means to put or to place.

In this sentence, Je mets un glaçon dans l’eau means I put an ice cube in the water.

French mettre is a very common verb and can be used in many situations, for example:

• mettre un livre sur la table = to put a book on the table
• mettre un manteau = to put on a coat
• mettre de l’eau à bouillir = to put water on to boil

So here it is the ordinary physical sense: placing something somewhere.

Un glaçon means an ice cube specifically.

La glace can mean:

• ice in general
• ice cream
• mirror, in some contexts

So if you mean a single cube of ice that you put in a drink or in water, un glaçon is the natural choice.

Examples:

• Je mets un glaçon dans mon verre. = I put an ice cube in my glass.
• Il y a de la glace sur la route. = There is ice on the road.

Because eau starts with a vowel sound, la becomes l’ before it. This is called elision.

So:

• la + eau becomes l’eau

French does this often with le and la before vowels or silent h:

• l’eau
• l’école
• l’homme

So dans l’eau simply means in the water.

For the same reason as l’eau: French avoids the clash of two vowel sounds.

Que becomes qu’ before a word beginning with a vowel or silent h.

So:

• parce que il fait chaud → parce qu’il fait chaud

This is standard French.

Other examples:

• Je pense qu’elle vient.
• Il faut qu’on parte.

This is just the normal French expression.

French often uses faire with weather and temperature expressions:

• il fait chaud = it is hot
• il fait froid = it is cold
• il fait beau = the weather is nice
• il fait mauvais = the weather is bad

The il here does not refer to a specific masculine thing. It is an impersonal subject, like it in it is hot.

So you should learn il fait chaud as a fixed expression.

Because in il fait chaud, chaud is part of an impersonal expression, not an adjective agreeing with a feminine noun.

You are not describing eau here. You are describing the general condition: it is hot.

So it stays:

• il fait chaud
• il fait froid

But if you were describing a feminine noun directly, then agreement would matter:

• L’eau est chaude. = The water is warm/hot.

That is different from Il fait chaud, which refers to the weather or overall temperature.

Parce que means because and introduces the reason.

So the sentence structure is:

• Je mets un glaçon dans l’eau = main idea
• parce qu’il fait chaud = reason

This is very common in French.

Examples:

• Je reste à la maison parce qu’il pleut.
• Elle mange parce qu’elle a faim.

Before a vowel, parce que becomes parce qu’, as in your sentence.

Yes. Je mets is in the present tense, and like the French present tense in general, it can cover both ideas depending on context:

• I put
• I am putting

French does not usually need a separate form like English am putting.

So Je mets un glaçon dans l’eau could mean:

• I put an ice cube in the water.
• I’m putting an ice cube in the water.

The exact meaning depends on the situation.

Because dans means in / into, which fits the idea of physically putting the ice cube inside the water.

• mettre quelque chose dans l’eau = put something in the water

À l’eau can appear in French, but it usually does not mean the same thing here. It can suggest ideas like into the water in a broader sense, or occur in set expressions, but for an object placed inside water, dans l’eau is the normal choice.

So this sentence correctly uses dans.

A careful pronunciation would be roughly:

Je mets un glaçon dans l’eau parce qu’il fait chaud.

Some helpful notes:

• je sounds like zhuh
• mets sounds like may
• un has a nasal vowel; there is no exact English equivalent
• glaçon sounds roughly like glah-son with a nasal final vowel
• dans also has a nasal vowel
• l’eau sounds like loh
• parce qu’il often flows together smoothly
• fait sounds like feh
• chaud sounds roughly like show

In normal speech, the sentence is quite fluid, especially around:

• dans l’eau
• parce qu’il

Yes. That would mean I put ice cubes in the water.

The difference is just singular vs. plural:

• un glaçon = one ice cube
• des glaçons = some / several ice cubes

So both are correct, depending on how many ice cubes you mean.