Le chou-fleur que j’ai acheté hier est encore bon, alors je vais le cuire avec les lentilles.

Chou-fleur is a fixed compound noun meaning cauliflower. It is masculine, which is why the sentence says le chou-fleur and later bon rather than bonne.

The hyphen is just part of the standard spelling. In the plural, it becomes choux-fleurs.

Here, que is a relative pronoun. It refers back to le chou-fleur and introduces extra information about it:

• Le chou-fleur = the cauliflower
• que j’ai acheté hier = that I bought yesterday

So que works like that, which, or sometimes nothing at all in natural English:

• The cauliflower that I bought yesterday...
• The cauliflower I bought yesterday...

J’ai acheté is the passé composé, a very common French past tense used for a completed action.

It is made of:

• j’ai = I have
• acheté = bought

So literally it looks like I have bought, but in normal English here it is usually translated as I bought.

Also, j’ai is just a contraction of je ai, which French does not allow, so it becomes j’ai.

Because the sentence includes hier (yesterday), the speaker is talking about a specific completed action in the past.

This is a very good question, because in fact yes, in principle it does.

With avoir, the past participle agrees with a preceding direct object. In this sentence, que stands for le chou-fleur, and it comes before acheté, so agreement applies.

However, le chou-fleur is masculine singular, and the masculine singular form is simply acheté, with no extra ending. So you do not see any visible change.

You would notice the agreement more clearly in examples like:

• La pomme que j’ai achetée = The apple that I bought
• Les fleurs que j’ai achetées = The flowers that I bought

Encore here means still, not again.

So est encore bon means is still good. When talking about food, bon often means:

• good
• fine
• still okay to eat

So the idea is that the cauliflower has not gone bad yet.

Also, bon is an adjective, which is why it is used after est. You would not use bien here, because bien is usually an adverb, not the normal adjective for describing food in this way.

The le is a direct object pronoun meaning it. It refers back to le chou-fleur.

Instead of repeating the noun, French replaces it with le:

• je vais cuire le chou-fleur
• je vais le cuire

So this works like English I’m going to cook it.

Because chou-fleur is masculine singular, the pronoun is le.

With aller + infinitive (the near future), object pronouns usually go right before the infinitive, not before aller.

So:

• je vais le cuire = correct
• je le vais cuire = not standard modern French

This is the normal pattern:

• Je vais le faire = I’m going to do it
• Nous allons les manger = We’re going to eat them

So in this sentence, le goes directly before cuire.

Je vais cuire is the near future: aller + infinitive.

It means I am going to cook or I’m going to cook.

French often uses this form for something the speaker plans to do soon or has just decided to do. It sounds natural here because the speaker is reacting to the situation:

• the cauliflower is still good,
• so they are going to cook it.

A simple future would also be possible in some contexts:

• je le cuirai

But je vais le cuire feels more immediate and conversational.

Les lentilles can sound like the lentils, meaning lentils that are already understood in the situation, or lentils as a food item in a general, familiar sense.

French uses the definite article more often than English does. So avec les lentilles can sound perfectly natural even where English might simply say with lentils.

If you said avec des lentilles, that would mean with some lentils, which is also possible, but slightly less specific.

So the choice here suggests the lentils are being thought of as a known part of the meal rather than just an unspecified amount.