Learn Hardcore French: Pour lui, c'est le meilleur système qu'ils aient trouvé, avec plus d'avantages que d'inconvénients. - For him, it is the best system they have found, with more advantages than disadvantages.

So it doesn’t mean the system is designed for his benefit or for his use specifically. It frames what follows as his personal judgment, not an objective fact shared by everyone.

Why is it c'est le meilleur système and not il est le meilleur système?

In French, c'est and il est are not interchangeable.

Use c'est before:

a noun or noun phrase:
- C'est le meilleur système. – It’s the best system.
- C'est un problème difficile.
a stressed pronoun:
- C'est lui.
most general identifications:
- C'est intéressant. (as a general comment)

Use il est mainly before:

an adjective referring to someone/something already clearly identified:
- Il est intéressant, ce système. – It is interesting, this system.
some set expressions (e.g. il est possible que, il est important de).

Because le meilleur système is a noun phrase, French requires c'est:

✅ C'est le meilleur système.
❌ Il est le meilleur système. (sounds wrong/unnatural)

Why is it le meilleur système and not something with mieux, like le mieux système?

The difference:

meilleur = better / best as an adjective
→ it modifies a noun
- un meilleur système – a better system
- le meilleur système – the best system
mieux = better as an adverb (or used alone as “better”)
→ it modifies a verb, an adjective, or stands on its own
- Ce système fonctionne mieux. – This system works better.
- Ça va mieux. – It’s better.

Since système is a noun, you must use the adjective meilleur, not the adverb mieux:

✅ le meilleur système
❌ le mieux système

Why do we use qu'ils aient trouvé instead of qu'ils ont trouvé?

Qu'ils aient trouvé uses the subjunctive, while qu'ils ont trouvé uses the indicative.

Grammar rule with superlatives
After a superlative like le meilleur, French often uses the subjunctive in the relative clause, especially when it’s expressing:
- a subjective judgment
- something not presented as an objective, checked fact

So:

C'est le meilleur système qu'ils aient trouvé.
→ In his view, it’s the best system they have found (of all the ones they might have found).

Indicative would be more factual
You can hear:

C'est le meilleur système qu'ils ont trouvé.

This sounds more like:

a neutral statement of fact: among the systems they actually found, this one is the best.

In your sentence, Pour lui already shows it’s his opinion, so the subjunctive (aient trouvé) fits well with that sense of subjective evaluation. It also sounds a bit more formal / careful than ont trouvé.

What tense is aient trouvé, and how is it formed?

Aient trouvé is the subjunctive past (passé du subjonctif) of trouver.

Formation:

Take the present subjunctive of avoir:
- que j'aie
- que tu aies
- qu'il / elle / on ait
- que nous ayons
- que vous ayez
- qu'ils / qu'elles aient
Add the past participle: trouvé

So:

qu'ils aient trouvé = that they have found

Meaning-wise, it’s like English “have found” inside a subjunctive frame:

le meilleur système qu'ils aient trouvé
→ the best system (that) they have found (in a subjective way)

Shouldn’t trouvé agree with système? Why is there no extra ending?

With avoir (including in the subjunctive), the past participle can agree in gender and number with a preceding direct object.

Here:

que refers to le système (masculine singular)
que is the direct object of aient trouvé
it comes before the auxiliary (aient)

So in theory the past participle does agree with système – but:

masculine singular agreement is just trouvé (no extra letter).
There’s nothing to change in writing.

This is why you see:

le meilleur système qu'ils aient trouvé (masc. sg. → trouvé)
but you would write:
- les meilleures solutions qu'ils aient trouvées (fem. pl. → trouvées)

Why is it plus d'avantages que d'inconvénients, and not plus des avantages que des inconvénients?

In French, after plus (and other quantity words like beaucoup, peu, moins, assez, trop), you generally use de + noun:

plus de
- noun
moins de
- noun
beaucoup de
- noun

So:

✅ plus d'avantages
❌ plus des avantages (wrong in this comparative/quantitative structure)

When you compare two quantities, you keep de before both nouns:

plus de A que de B

So the correct structure is:

plus d'avantages que d'inconvénients
→ literally: more of advantages than of disadvantages
→ idiomatically: more advantages than disadvantages

Using des here would break that standard quantitative/comparative pattern.

Can avantages and inconvénients be used like advantages and disadvantages in English?

Mostly yes.

un avantage → an advantage / a benefit
un inconvénient → a disadvantage / a drawback

Common patterns:

les avantages et les inconvénients – the pros and cons
Ce système a des avantages, mais aussi des inconvénients.

Subtle nuance:

inconvénient sometimes has the flavor of a practical drawback, inconvenience, downside, not necessarily something morally bad.

What’s the difference between pour lui, selon lui, and à son avis in meaning and tone?

All three mean roughly “in his opinion”, but with slight nuances:

pour lui
- very common, neutral
- emphasizes his personal perspective
- literally: for him
- used a lot in everyday speech and writing
selon lui
- literally: according to him
- can sound a bit more detached / report-like
- often used when reporting someone’s view:
  - Selon lui, le système est efficace. – According to him, the system is effective.
à son avis
- literally: in his opinion
- explicitly marks this as an opinion, not a fact
- similar tone to English in his opinion, maybe slightly more insistent that this is subjective.

In your sentence, Pour lui nicely introduces the idea that everything that follows is his personal assessment, which matches the use of le meilleur and the subjunctive aient trouvé.

Could the position of pour lui change, like C'est, pour lui, le meilleur système…? Would that change the meaning?

Yes, you can move pour lui, and the meaning stays basically the same, but the focus changes a bit.

Possible positions:

Pour lui, c'est le meilleur système qu'ils aient trouvé…
- very natural
- puts his point of view up front
C'est, pour lui, le meilleur système qu'ils aient trouvé…
- also correct but sounds more insistent / written / stylistic
- the commas add a slight pause: “It is, for him, the best system…”
C'est le meilleur système qu'ils aient trouvé pour lui.
- now pour lui risks being understood as “for him (for his use/benefit)” rather than “in his opinion”
- so this last position is not good if you want to express his opinion; it starts to sound like the system is best suited to him personally.

The original version (Pour lui, c'est…) is the clearest way to mean “in his view”.