Game in flash 8 better jump and fall from land

1

hi im sorry for my english.i need complete my flash game and i have one problem with jump character on land and falling from them.
this is code.:

onClipEvent (load) {
    y_start = _y;
    skok = false;
    rychlostskoku = 0;
}
onClipEvent (enterFrame) {
    if (Key.isDown(Key.RIGHT)) {
        gotoAndPlay(this._currentFrame+1);
        this._x += 5;
        this._xscale = 100;
    }
    if (Key.isDown(Key.LEFT)) {
        gotoAndPlay(this._currentFrame+1);
        this._x -= 5;
        this._xscale = -100;
    }
    if (skok) {
        _y += rychlostskoku;
        rychlostskoku += 1;
        if (_y>=y_start) {
            _y = y_start;
            skok = false;
        }
    } else {
        if (Key.isDown(Key.SPACE)) {
            skok = true;
            rychlostskoku = -13;
        }
    }
    if (hitTest(_root.zed) == true) {
        _x = _x-5;
    }
    if (hitTest(_root.zed2) == true) {
        _x = _x+5;
    }
    if (hitTest(_root.plosinka) == true) {
        _y = 311;
        y_start = _y;
        if (_x<=332.9) {
            _y = 395;
        }
        if (_x>=444.9) {
            _y = 395;
        }
    } else if (hitTest(_root.plosinka1) == true) {
        _y = 255;
        y_start = _y;
        if (_x<=158) {
            if (hitTest(_root.plosinka) == false) {
                _y = 395;
            } else {
                _y = 311;
            }
        }
        if (_x>=270) {
            if (hitTest(_root.plosinka) == false) {
                _y = 395;
            } else {
                _y = 311;
            }
        }
    } else {
        y_start = 395;
    }
}