Difficulty starting a vehicle when cold can be caused by several factors. Here are some common issues that can contribute to hard cold starts in a 2006 Honda Civic:

1. **Weak Battery**: A weak or aging battery can struggle to provide enough power to start the engine, especially in colder temperatures when the engine requires more power to turn over. Consider checking the battery's age and performance, and if needed, have it tested and possibly replaced.

2. **Faulty Fuel System**: Issues with the fuel system, such as a clogged fuel filter or a weak fuel pump, can result in poor fuel delivery to the engine during startup. This can cause difficulty in starting the vehicle, especially when cold. A mechanic may need to inspect and potentially replace these components.

3. **Faulty Spark Plugs**: Worn-out or fouled spark plugs can prevent the engine from starting smoothly, particularly in colder conditions. Consider inspecting the spark plugs for wear and replacing them if necessary.

4. **Faulty Ignition System**: Issues with the ignition system components, such as the ignition coil or ignition switch, can cause starting problems. A professional diagnostic may be needed to identify specific faulty components.

5. **Coolant Temperature Sensor**: A faulty coolant temperature sensor can incorrectly relay information to the engine control unit, leading to incorrect fuel mixture during startup. This can result in difficulty starting the vehicle when cold. Replacing the sensor may be necessary.

6. **Air Intake Issues**: Clogged air filters or issues with the air intake system can affect the air-fuel mixture going into the engine, impacting startup performance. Inspect and replace air filters as needed.

It's recommended to have a qualified mechanic diagnose the specific cause of the hard cold start issue in your 2006 Honda Civic to accurately address and resolve the problem.