Hier, j’ai acheté un bonnet en coton parce que mon vieux bonnet en laine était trop chaud.

Hier means yesterday, and it is very commonly placed at the beginning of the sentence to set the time frame right away.

• Hier, j’ai acheté un bonnet...
• You could also say J’ai acheté un bonnet hier, which is also natural.

Starting with Hier is a little like saying Yesterday, I bought... in English.

J’ai acheté is the passé composé, which is the tense French usually uses for a completed action in the past.

Here, buying the hat is a single finished event:

• Hier, j’ai acheté... = Yesterday, I bought...

Compare:

• j’ai acheté = I bought / I have bought
• j’achetais = I was buying / I used to buy

So j’ai acheté is the right choice because the purchase happened once and was completed yesterday.

French avoids putting je directly before a vowel sound. So je becomes j’ before a word starting with a vowel or silent h.

• je ai → j’ai
• je aime → j’aime
• je habite → j’habite

This is called elision.

Not exactly. In French, un bonnet usually means a knitted hat, beanie, or winter cap worn on the head.

This is a classic false friend:

• French un bonnet = a woolly hat / beanie
• English bonnet = usually a type of hat for babies or women, or the hood of a car in British English

So in this sentence, un bonnet en coton is a cotton beanie / cotton hat.

Here en means made of or in a material.

• un bonnet en coton = a cotton hat / a hat made of cotton
• un bonnet en laine = a wool hat / a hat made of wool

French often uses en + material where English might use the material directly as an adjective:

• une table en bois = a wooden table
• une bague en or = a gold ring

Most French adjectives come after the noun, but some common short adjectives often come before it. Vieux is one of them.

So:

• mon vieux bonnet = my old hat

Other common adjectives that often go before the noun include:

• beau (beautiful)
• jeune (young)
• petit (small)
• grand (big/tall)
• bon (good)
• mauvais (bad)

This is part of a group often remembered by learners as BAGS or similar adjective-position patterns, though French has exceptions.

Bonnet is actually masculine: un bonnet.

So the possessive adjective must match the noun:

• mon bonnet = my hat
• ton bonnet = your hat
• son bonnet = his/her hat

The gender of the noun is something you have to learn with the word itself. The ending can sometimes help, but it is not always reliable.

Était is the imparfait form of être, and it is used here because it describes an ongoing state in the past.

• mon vieux bonnet en laine était trop chaud = my old wool hat was too warm

This is a description, not a single event. French often uses:

• passé composé for a completed action: j’ai acheté
• imparfait for background, description, or ongoing states: était trop chaud

So the sentence mixes the two very naturally:

• completed action: I bought a new hat
• background reason: the old one was too warm

Parce que means because and introduces a full clause with a subject and verb.

• parce que mon vieux bonnet en laine était trop chaud

It is the most direct equivalent of English because.

Compare:

• parce que
  • full clause
    
    • ...parce que mon bonnet était trop chaud
• à cause de
  • noun
    
    • ...à cause de la chaleur

Car can also mean for/because, but it is often a bit more formal or literary. In everyday speech, parce que is usually the most common choice.

French simply repeats bonnet:

• un bonnet en coton
• mon vieux bonnet en laine

That is completely natural. English sometimes uses one to avoid repetition:

• I bought a cotton hat because my old wool one was too warm.

French does not need an equivalent word here. Repeating the noun is normal and often preferred.

• chaud = warm / hot
• trop chaud = too warm / too hot

The word trop means too much or too in the sense of excess.

So:

• Il fait chaud = It’s warm / hot.
• Il fait trop chaud = It’s too warm / too hot.
• Ce bonnet est trop chaud = This hat is too warm.

In this sentence, the old wool hat was uncomfortable because it was excessively warm.

Adjectives agree with the noun they describe. Here, chaud describes bonnet, which is masculine singular.

• un bonnet → chaud
• une veste → chaude

So:

• Le bonnet est chaud.
• La veste est chaude.

That is why the sentence has était trop chaud, not chaude.

It is not always strictly required, but it is very common and helpful when a time expression comes at the beginning of the sentence.

• Hier, j’ai acheté un bonnet...

The comma makes the sentence easier to read and marks Hier as an introductory element. In informal writing, some people may leave it out, but using it is a good habit.