Learn French: Pour identifier celui qui est sous le chêne, je vérifie l'odeur. - To identify the one that is under the oak, I check the smell.

Celui is a demonstrative pronoun meaning "the one". We use it to replace a specific noun (masculine singular) so we don't have to repeat it. Qui means "that" or "who" and acts as the subject of the verb est. Together, celui qui means "the one that". If we used ce qui, it would mean the abstract "what" or "that which", rather than a specific physical object.

Could I say celle qui instead of celui qui?

Yes, but it depends on the noun you are replacing. Celui is masculine singular, so the speaker is talking about a masculine object (like a mushroom, un champignon). If they were talking about a feminine object, like a plant (une plante), they would use the feminine form celle qui.

Why does the sentence start with Pour identifier instead of just Identifier?

When the English "to" means "in order to" before a verb, French requires the preposition pour followed by the infinitive. So Pour identifier means "In order to identify" or simply "To identify".

I see l'odeur in the sentence. Is odeur masculine or feminine?