Ceux-ci sont les meilleurs modèles, selon la vendeuse, parce qu’ils résistent mieux à la pluie.

Ceux-ci is a demonstrative pronoun. It means these ones or these, and it replaces a plural noun that is already understood from the context.

In this sentence, it stands for something like these models without repeating the noun immediately.

A few useful points:

• ceux = masculine plural those/these ones
• -ci adds the idea of this/these here or sometimes the ones just mentioned
• the hyphen is normal in forms like ceux-ci, celle-là, celui-ci, etc.

Both are possible in French, but they suggest slightly different things:

• ceux-ci = these ones / the latter / the ones here
• ceux-là = those ones / the former / the ones there

In real modern French, the contrast is not always very strict. Native speakers often use ceux-là quite a lot, even when English might say these. But ceux-ci is still correct and often used when the speaker is pointing to the nearer items or making a neat contrast.

Because ceux-ci is plural.

• ceux-ci = these ones
• plural subject → plural form of être
• so: ceux-ci sont

Compare:

• celui-ci est = this one is
• ceux-ci sont = these ones are

Because meilleur is the form used with nouns, while mieux is usually used with verbs.

Here, meilleurs describes the noun modèles, so you need the adjective meilleur.

• un meilleur modèle = a better model
• les meilleurs modèles = the best models

But with a verb, you use mieux:

• ils résistent mieux = they resist better

A very useful shortcut is:

• meilleur ↔ adjective, linked to bon
• mieux ↔ adverb, linked to bien

Because it agrees with modèles, which is masculine plural.

Agreement here works like this:

• modèle = masculine singular
• modèles = masculine plural
• so the adjective must also be masculine plural:
  • meilleur → masculine singular
  • meilleurs → masculine plural

So:

• le meilleur modèle
• les meilleurs modèles

In this sentence, it is a superlative: the best models.

Why? Because of les:

• de meilleurs modèles = better models → comparative
• les meilleurs modèles = the best models → superlative

So meilleur can mean better or best, depending on the structure around it.

Selon la vendeuse means according to the saleswoman / according to the salesperson.

It tells you that this is the seller’s opinion or claim, not necessarily an absolute fact. So the sentence is not just saying these are the best models; it is saying these are the best models, according to the seller.

This is a very common use of selon.

Examples:

• Selon Marie, c’est une bonne idée.
• Selon le journal, il va pleuvoir.

Because it is a parenthetical comment: extra information inserted into the main sentence.

The core sentence is:

• Ceux-ci sont les meilleurs modèles parce qu’ils résistent mieux à la pluie.

The phrase selon la vendeuse is added to show the source of the opinion. The commas help mark that interruption.

English often does the same thing:

• These are the best models, according to the salesperson, because...

Because vendeuse is the feminine form, used for a female seller or sales assistant.

• un vendeur = a male salesperson
• une vendeuse = a female salesperson

So la vendeuse tells you the person is female.

Because French normally avoids que directly before a vowel sound. So que becomes qu’.

This is called elision.

• parce que
  • ils
• becomes parce qu’ils

You see the same thing in many other places:

• que elle → qu’elle
• que on → qu’on
• je ai → j’ai

Ils refers to les modèles.

Even though models are things, not people, French still uses ils for masculine plural nouns.

So here:

• les modèles = masculine plural
• pronoun replacing it = ils

This is completely normal in French.

Because it goes with ils.

The verb is résister in the present tense:

• je résiste
• tu résistes
• il / elle résiste
• ils / elles résistent

Since the subject is ils (referring to les modèles), the correct form is résistent.

Because mieux is an adverb, and it modifies the verb résistent.

It tells us how they resist: they resist better.

This is different from meilleur, which modifies a noun.

Compare:

• de meilleurs modèles = better models
• ils résistent mieux = they resist better

So in one sentence you get both forms:

• meilleurs with the noun modèles
• mieux with the verb résistent

Because the verb résister normally takes the preposition à.

So you say:

• résister à la pluie
• résister au froid
• résister à l’eau
• résister à la pression

That is just the standard pattern of the verb.

So ils résistent mieux à la pluie means they stand up better to rain / in rainy conditions.