Queries pertaining to grammatical concepts

Question

Bonjour Madame Cécile !I have been attempting tests at Kwiziq and have faced a few problems which I would like to ask here-1. C’est le mois d’avril et il a commencé ______ faire très chaud. Here, would the answer be “à/de” ? How to judge when to use “commencer à / “commencer de” ?2. Regarde cet arbre sans fruits. Mais _________ qui est devant nous en a beaucoup.Here, which of the following is grammatically accurate -  celui-ci /celui ? I opted for “celui-ci” but the answer was “celui”. Why, Madame ?3.J’ai vu quelque chose dans ton sac.To form the negative of the sentence which is correct -Je n’ai vu rien/rien vu dans ton sac. This is not highlighted in the lesson- https://french.kwiziq.com/my-languages/french/review/623/26678024. Ces hommes parlent sans cesse.To form a question would we write-Qui parle/ Qui parlent  sans cesse ?Merci encore Madame Cécile pour votre aide continuel.Je vous souhaite une bonne journée!

Accepted Answer

Hi Shrey, 
1. The difference between 'commencer à' et 'commencer de' is very subtle but I will try to explain.
You would use 'commencer à' when the action is just the start and that there will be a progression in the future -
L' enfant a commencé à parler à l'âge de quatre ans = The child started to speak at the age of four.
J'ai commencé à apprendre l'anglais à l'école primaire = I started learning English at primary school 
Elle commence à s'intéresser au français = She is starting to be interested in French 
But you will use 'commencer de ' when the action it describes, is limited -
Le Président a commencé de parler à midi = The chairman started speaking at 12 noon
Je commence de manger avant ton arrivée = I am starting to eat before you arrive
Having said that sometimes the two verbs are used wrongly and the 'à' version seems more popular.
2. Celui is right because it means the one /the tree  rather than that one :
Celui qui est devant nous en a beaucoup 
3. it is -
Je n'ai rien vu dans ton sac 
4. I have already answered elsewhere
Hope this helps!
p.s. 'aide continuelle '