Qu'est-ce qu'il faut ajouter avant "on" apres les mots si, et,ou,qui,que,ou,quoi?

First of all, it was not easy finding a good explanation about your first question.

Q: Why is there an l' in phrases like ce que l'on dit ("what people say") and si l'on veut ("if one wants to")?
A: The easy answer is that the l' is there for "euphonic" reasons -- to make things easier to pronounce: que l'on, si l'on, où l'on, etc. are more fluid than qu'on, si on and où on. The more complicated answer is that this l' is left over from a definite article in historical French. Please see the sites below for an explanation of the etymology.

Q: Can we translate the l' using the word "it"? Does it refer to something else in the sentence? Does it have a grammatical function?
A: No, the l' does not mean "it." As a matter of fact, it doesn't have any meaning at all. You can simply ignore the l' when translating. It does not have a grammatical function in modern French, and it does not refer to anything else in the sentence.

Q: Is it "better" to include the l'?
A: The l' is often omitted in casual speech, but it is generally included in proper written French.

Now, for the agreement of the past participle, conjugated with être.
il est descendu (no aagreement)
ils sont descendus (it reflects that "ils" is plural)
elle est descendue (feminine singular, just like "elle")
elles sont descendues (feminine plural, just like "elles"

As for "on" = on est descendu (treated as if it were masculine singular) The pronoun "on" is in indefinite pronoun (neither masculine nor feminine.)

Sra (aka Mme)

I'll flag this in case you have another question.

I'll send this to one of our French experts SraJMcGin.

Thank you.

C'est pour le son: l'on

(après avec accent grave) où aussi.

Sra (aka Mme)

thank you. can i ask you another question

Donnez-moi un exemple des accords qu'on fait avec il,elle,nous, ils et elles au passe compose avec "etre" et les pronom suject "on".

My teacher gave an example.. and i did my own but im not sure if it is correct

Donnez-moi un exemple des accords qu'on fait avec il,elle,nous, ils et elles au passe compose avec "etre" et les pronom suject "on".

My teachers example: (ils) On est venus
He said we can use the same verb
So what i did:
(il) On est venu
(elle) On est venue
(nous) On est venus
(elles) On est venus
Im not sure if it is correct

Just the last one is both feminine and PLURAL = Elles sont venues

Oh, oh, I see you got the past participle on the other 3 but NTOT the fact that "nous" is also plural and the auxiliary/helping verb must agree with that subject = nous sommes venus (and this assuming that "nous" is NOT all feminine but either all masculine or a mixed group.

By the same verb, he meant être in some form to agree with the subject.

Sra (aka Mme)

Avant "on", il faut ajouter un "t" apostrophe ('). Par exemple, si on veut dire "si on" en français, on écrit "si + ' + on". De même, pour les autres mots, on utilise la même règle. Par exemple, et on devient "et + ' + on", ou on devient "ou + ' + on", qui on devient "qui + ' + on", que on devient "que + ' + on", ou on devient "ou + ' + on", quoi on devient "quoi + ' + on". Cela permet de fusionner les deux mots en les prononçant comme un seul mot dans la langue française.