How could I check the actual stock after selling any goods?

I added an inventory with a product 70 pcs qty. Next, I sold 30 pcs from there. But when checking the inventory from the report option the qty. shows the same (70 pcs). But it should be 40 pcs (Example: 70-30=40). How could I see the actual qty after selling goods from the stocked goods? Please help anyone by answering this.

Have you read the guide about inventory items?

BTW, placing screenshots of the edit pages would make it easier to understand what the exact problem is.

How did you add the 70 items to your stock?
How did you sell the items?
How have you defined your inventory items?

Manager has mulitiple features to help manage inventory and how you set it up and use it will determine when inventory stock levels are modified

As Mark said, the guides are full of information about how to manage inventory and should be your first port of call