Je mets ma capuche parce qu’il commence à pleuvoir et qu’on entend déjà le tonnerre.

Because the verb is mettre and in the present tense it conjugates as:

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

So with je, you need mets.

Here, mettre means to put on in the sense of clothing.

So Je mets ma capuche means I’m putting on my hood or I put my hood up.

French often uses mettre for putting on clothes or accessories:

• mettre un manteau = to put on a coat
• mettre un chapeau = to put on a hat

French often uses a possessive adjective like mon / ma / mes when talking about clothing, especially when the ownership is clear and natural.

So:

• ma capuche = my hood

This is more straightforward here than la capuche.
A learner might expect French to avoid possessives more often, but with clothing, both patterns exist depending on the structure.

For example:

• Je mets ma veste. = I put on my jacket.
• Il lève la main. = He raises his hand.

With body parts, French often prefers the definite article in reflexive structures, but here capuche is treated as a personal item of clothing, so ma is natural.

Because in French, que becomes qu’ before a vowel sound.

So:

• que il becomes qu’il

This is called elision. French avoids certain vowel clashes this way.

You see the same thing in:

• que elle → qu’elle
• que on → qu’on

So parce qu’il commence à pleuvoir is just the normal shortened form of parce que il commence à pleuvoir.

Because the sentence is linking two clauses that both depend on parce que:

• parce qu’il commence à pleuvoir
• et qu’on entend déjà le tonnerre

In English, we might say:

• because it’s starting to rain and we can already hear thunder

In French, it is very common to repeat que before the second verb phrase after et.

So the structure is essentially:

• because it’s starting to rain
• and because we can already hear thunder

Even though English usually does not repeat because, French often repeats que in this kind of coordination.

Grammatically, on can mean one, but in everyday French it very often means we or people in general.

Here, qu’on entend déjà le tonnerre most naturally means:

• and we can already hear the thunder or
• and you can already hear the thunder

It does not sound formal like English one in this sentence.

Very common spoken French uses:

• On va au cinéma. = We’re going to the movies.
• On entend le tonnerre. = We can hear the thunder.

Because commencer is followed by à when another verb comes after it.

So:

• commencer à + infinitive

Examples:

• commencer à manger = to start eating
• commencer à parler = to start speaking
• commencer à pleuvoir = to start raining

So il commence à pleuvoir literally means it is starting to rain.

French usually uses an article more often than English does.

So where English may say:

• hear thunder

French normally says:

• entendre le tonnerre

The definite article le is natural here. French often treats weather phenomena and general nouns this way.

Compare:

• J’aime le chocolat. = I like chocolate.
• On entend le vent. = We can hear the wind.
• On entend le tonnerre. = We can hear thunder.

Déjà means already.

It shows that the thunder can be heard at this point already, which suggests the rain is imminent or has begun to develop.

So:

• on entend déjà le tonnerre = we can already hear the thunder

It adds a sense that the signs of the storm are already present.

Not really in a meaningful sense. It is an impersonal il.

In weather expressions, French uses il the way English uses it:

• il pleut = it’s raining
• il neige = it’s snowing
• il commence à pleuvoir = it’s starting to rain

This il does not refer to a person or object. It is just required by the grammar.

Quite natural already. In speech, a few things are worth noticing:

• qu’il sounds linked together
• qu’on entend also links smoothly
• on entend may sound like on-nentend because of liaison in careful speech, though not everyone emphasizes it strongly

Also, in everyday spoken French, on is extremely common instead of nous, so this sentence sounds very natural and conversational.

Yes, depending on the situation.

• Je mets ma capuche = I put on / put up my hood
• Je remonte ma capuche = I pull my hood up again / I raise my hood

Mettre is broad and neutral.
Remonter adds the idea of pulling it upward, often because it was down before.

So the original sentence is simple and natural, but other verbs are possible depending on the exact action.

Because French often uses the present tense for actions and situations happening right now.

Here the sentence describes a current action with a current reason:

• Je mets ma capuche = I’m putting my hood up
• il commence à pleuvoir = it’s starting to rain
• on entend déjà le tonnerre = we can already hear thunder

English often uses the progressive form, but French usually just uses the simple present for this kind of immediate situation.