« L'objet qu'est le soleil » : pourquoi pas « qui » ?

Question

Je maitrise les bases au sujet des pronoms relatifs qui vs que; je ne m'en enquiers pas ici. Hélas, je ne comprends pas l'utilisation qui en est faite dans cette réponse de Gilles. En première estimation j'aurais employé qui au lieu de que.

Il est rare d'utiliser dans le soleil autrement que pour parler de ce qui se passe dans l'objet qu'est le soleil ...

Dans son commentaire, il écrit ce qui suit, mais comme je connais à peine les termes de grammaire en français, je ne comprends pas ce commentaire.

Oui, que est ici un pronom relatif. Il est attribut du sujet dans la subordonnée, donc on utilise que : qui est utilisé seulement quand le pronom est sujet.

Answer 1

I'm going to answer in English to make it simpler for you.

User 'Gilles' 's answer translates as:

Yes, que is here a relative pronoun. It replaces the predicative (1) in the relative clause (1), so que is used. Qui when the pronoun represents a subject.

More simply put, the relative pronouns in French are separated by the function of the noun to which the relative pronoun is appended in that sentence (since the pronoun takes the place of said noun in the relative clause).

• Qui is used when that noun is a subject: l'homme qui me regarde. ("the man who's looking at me")
• Que is used when it is an object or an attribut: l'homme que je regarde ("the man who I am looking at")
• Où is used when it is a locative: la ville où je suis née ("The place where I was born")
• Dont will show up if it a prepositional phrase introduced by de: l'homme dont je te parle ("The man who I am talking about")

(1) "Predicative" seems a very technical term in English grammar, but is very common in French grammar teaching. It mostly refers to the "object" of être and other copula-like construction (e.g. sembler).

(2) In French, most non-main clauses are referred to as subordonnée, but I think english grammar tends to separate them more finely.

Answer 2

I thought to add a summary to the other excellent answer in English. The key problem was my neglect of the subject-verb inversion in the relative clause. To wit, I thought that

1. l'objet qu'est le soleil

meant the incorrect 1.1, rather than the correct 1.2.

✘ 1.1. 'the object that IS the sun' ✘
✓ 1.2. 'the object that the sun IS'.✓

In 1.1, 'the object' (l'objet) is the Subject, and 'the sun' (le soleil) is the Predicate Noun or Predicative Nominative.
The Lexical Categories in 1.1 are reversed for 1.2.
In 1.2, 'the sun' (le soleil) is the Subject, and 'the object' (l'objet) is the Predicate Noun or Predicative Nominative.

My Key Problem: English does not allow Subject-Verb Inversion for 1.2, but I forgot that French does! So translate 1.2 into French:

1.3. l'objet que le soleil est

and finally apply Subject-Verb Inversion to the bolded in 1.3:

1. l'objet qu'est le soleil.