Place a section below the header

I have a problem .. I have a header and below I want to place a section, but when creating the section, it is placed on top of the page in front of the header. I have a nav created that I want to be ahead, so I used the z-index (I guess the problem of the section has to do with the z-index, right?).

Thank you.

nav{
    width: 1000px;
    height: 75px;
    background-color: #44acde;
    margin: auto;
    text-align: center;
    z-index: 1;
    position: absolute;
    left: 50%;
    transform: translate(-50%);
    padding: 10px;
    border-bottom: 700px;
}

nav ul {
    display: flex;
    margin: auto 0;
    padding: 20px;
    flex-direction: row;
    flex-wrap: wrap;
    justify-content: space-between;
}

 nav li {
     list-style: none;
 }


nav a{
    color: #fff;
    font-size: 20px;
    text-decoration: none;
    padding: 10px;
}


nav a img{
    height: 50px;
    margin-top: 0px;
}

/*-----------------------HEADER-----------------------*/

header{
    height: 1000px;
    width: 100%;
    background-color: #071d56;
    z-index: 0;
    margin-top: 0px;
    position: absolute;
    display: block;
}

header img{
        margin-top: 200px;
}


#video_background {
    position: absolute;
    bottom: 0px;
    right: 0px;
    min-width: 100%;
    min-height: 100%;
    width: auto;
    height: auto;
    z-index: -1000;
    overflow: hidden;
    opacity: .5;
}



/*-----------------------MAIN-----------------------*/


#main{
    height: 400px;
    width: 100%;
    max-width: 1000px;
    background-color: red;
    position: absolute;
    z-index: 0;

    text-align: center;
    display: block;
}