2.9 understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

In a circuit, the voltage is the driving force.

Any component added into the circuit (for example, a lamp) will increase the amount of resistance in the circuit, this will decrease the current.

The voltage is trying to push the current around the circuit and the resistance is trying to slow it down, therefore, the relative sizes of the voltage and resistance decide how big the current will be.

If you increase the voltage - more current will flow
If you increase the resistance - less current will flow (alternatively, if you want the same amount of current to fly when the resistance is increased, you could increase the voltage).