My toyota corolla is always hard to start whenever it is cold day?how can i fix it?

Yourr car have 4 cylinders, there are 4 injectors and 1 cold start injector. You can trace it from your Fuel distributor. Your cold start is operated by a electric valve. It has a normal close state and only open when you start the engine. Its function to inject extra fuel so you can have a rich mixture for easy start. Now it is dead so you don't have a fast start. Have it replaced then you will be happy.