[dleviwing]

undef00;
The speed of light is different when moving through glass, water, or other transparent mediums. The speed of light is also different in a vacuum at the surface of the Earth than in the vacuum of space. However when we measure the speed of light in these vacuums, we obtain the exact same value.
Thus the speed of light is not constant, only the measurement of it in a vacuum is constant. We can view this as saying that light moves through the same quantity of spatial density for any given time increment. Or you can use Relativity.

I find your English very good. No need to be self-conscious. I have trouble with the English language also and it is the only language I speak.