notebook
The French word 'cahier' is used to refer to a notebook or a book of blank pages for notes. Much like in English, 'cahier' can be used in a variety of contexts, such as school, work, or personal journaling.
I need to buy a new notebook.
The speaker expresses a need to purchase a new notebook, possibly for school or work. The noun 'cahier' is the object that the speaker intends to buy.
Your notebook is on the table.
The speaker is telling someone that their notebook, or 'cahier', is located on the table. This is a common use of 'cahier' where it is the subject of the sentence.
She writes in her notebook every day.
The speaker describes a habitual action of a female subject, who writes in her notebook or 'cahier' on a daily basis. Here, 'cahier' is part of the prepositional phrase that indicates where the action of writing takes place.