x = new List<float>();
            y = new List<float>();
            for (int i = 0; i < 40; i++)
            {
                Random random = new Random();
                x.Add((float)random.NextDouble() * 100);
                y.Add((float)random.NextDouble() * 100);
            }

what i'm trying to do is:
cloud[i].x += 0.5f;

"Error    1    Cannot modify the return value of 'System.Collections.Generic.List<WindowsGame1.Game1.Cloud>.this[int]' because it is not a variable    J:\C Sharp\Cloud Generator\WindowsGame1\Game1.cs    149    21    WindowsGame1
"

public struct Cloud
        {
            public float x, y;
         }
public void Update()
{
        cloud[i].x += 0.5f;
}
"Error    1    Cannot modify the return value of 'System.Collections.Generic.List<WindowsGame1.Game1.Cloud>.this[int]' because it is not a variable

        public struct Cloud
        {
            public float x, y;  //represents approximate center of cloud, all cloud's particles are moved around it
            public List<double> xList, yList; //contains the location of every particle in the cloud. is fed a random list in the constructor
            public Cloud(float x1, float y1, List<double> xList1, List<double> yList1)
            {
                x = x1; //center X of cloud
                y = y1; // center y of cloud
                xList = xList1; //location X of individual particles
                yList = yList1; //location Y of individual particles
            }
        }
(update)
            for (int cloudIndex = 0; cloudIndex < clouds.Count; cloudIndex++)  //cloudIndex represents each cloud 'clump', a group of cloud particles
            {
                for (int particleIndex = 0; particleIndex < clouds[cloudIndex].xList.Count; particleIndex++)  // particleIndex manages each individual texture of each cloud
                {
                    clouds[cloudIndex].x += (0.5f);  //add 0.5 to the X pixel location of each cloud
                    clouds[cloudIndex].y += (0.5f); // same for Y
                    if ((clouds[cloudIndex].yList[particleIndex] + clouds[cloudIndex].y) > 750) //see next line
                        clouds[cloudIndex] = new Cloud(-200, random.Next(-200,500), randomList(50), randomList(50));  //generates a new cloud if current one goes off screen
                    if ((clouds[cloudIndex].xList[particleIndex] + clouds[cloudIndex].x) > 950) // same as above
                        clouds[cloudIndex] = new Cloud(random.Next(-200, 700), -200 , randomList(50), randomList(50)); //same
                }
            }