bag
The word 'sac' in French is used to refer to a bag, specifically a bag you would carry items in, such as a handbag, shopping bag or a sack. For example, 'sac à main' is a handbag, and 'sac de courses' is a shopping bag.
Where is my bag?
This sentence is typically used when someone is looking for their bag. It is in the present tense and the word 'Où' is used to ask 'where' in French.
He left his bag at school.
This sentence is typically used to denote that someone, in this case a male subject, has left their bag at school. The verb 'a laissé' is past tense, indicating a completed action.
The bag is heavy.
This sentence is used to describe the weight of the bag. In French, 'lourd' means heavy. The sentence is constructed like most simple sentences in English, with the subject (The bag), verb (is) and the adjective (heavy).