As forms are molded and organized, space is created. When space is organized, enclosed, and captured, architecture is created. So, without forms, space cannot be defined; and without the two working together, architecture cannot come into being.
Picturing kitchen layouts helps to understand how forms and space work together. The cabinets, island, table, chairs, i.e. the forms determine the space of the kitchen. The kitchen in the photo below shows how those forms as well as the wall planes define the space and further break it up.
Photo found at: http://www.elledecor.com/image/tid/5476?page=11&pause=1
The diagram below simplifies how the kitchen would look from above. Notice how the black shapes (which represent the forms in the kitchen) dictate the space outline of the kitchen.
DEFINING SPACE WITH HORIZONTAL ELEMENTS
Spaces can be defined by different types of horizontal planes. Think of sitting on a blanket in the grass for a picnic. The edges of the blanket define the space and boundaries where you keep your basket, food, etc. The space can be further defined with a contrasting plane, such as a white blanket on the green grass rather than a green blanket. The blanket would be considered a base plane
Other ways to define spaces using horizontal planes include a elevated base plane (e.g. a stage), a depressed base plane (e.g. an in-ground pool), and an overhead plane which is discussed below.
The interior space in the picture below is a great example of the use of an overhead plane defining space. Traditionally, we see white ceilings in residential spaces and draw little attention to them. However, in this space, the overhead plane is obvious and its surface articulation helps to define the space below even more.
Photo found at: http://www.elledecor.com/image/tid/6244?page=0&pause=1
The diagram below shows how the overhead plane works with the ground plane to create a space in between:
DEFINING SPACE WITH VERTICAL LINEAR ELEMENTS
Just as horizontal elements define spaces, vertical elements also do. Vertical elements and planes have a strong impact on how we perceive and relate to the spaces around us... more so than horizontal elements because vertical elements limit the distance we can see and move. For example, a space with four vertical walls not only contains us more, but also seems to be more defined than a space than that with just a ceiling and floor. So, obviously vertical elements are extremely important in interior design as they set boundaries for what is the exterior and what is the interior, for one.
The picture below illustrates how vertical planes and elements (the short walls and adjacent columns) help to define spaces within spaces: they separate the work and living spaces which otherwise, would be in one space.
Photo found at: http://www.elledecor.com/image/tid/6244?page=2&pause=1
The diagram below illustrates how the vertical elements in this space would appear from above:
This example shows two ways vertical planes define spaces. First, the four planes (walls) create a closed space. Second, the two planes (the combination of the short walls and columns) are parallel to two of the main planes creating a space within them.
OPENINGS IN SPACE-DEFINING ELEMENTS
The space in the picture above also illustrates another way form and space work together with openings between planes. If you think of the short walls and columns starting as a one continuous plane and then an opening spanning from the floor to the ceiling is cut out creating two planes, the two adjacent spaces are then connected. The opening lessens the definition of the spaces. Openings between planes can be vertical (as in this case), or horizontal; on the ceiling or on the floor.
Openings can also be at the corners of a space. When this happens the space takes on a diagonal direction and also erode the definition of the space. Openings can also be within planes, such as a cut-out in the wall like in the space in the picture below.
Photo found at: http://www.elledecor.com/image/tid/5476?page=6&pause=1
The diagram below shows how an opening within a plane creates a focal point, dictates the form around it, and organizes the spaces that are now connected.
QUALITIES OF ARCHITECTURAL SPACE
As you might suspect at this point, the architectural qualities of a space help to define form and space. They help to determine the degree of enclosure, view, and light qualities of a space.
A space with little or no openings is well defined and has a high degree of enclosure. When a space does have an opening, a view or focal is created and how the forms and spaces around it are organized is key. Also, openings reveal light whether from outside or from another room. Light illuminates forms, surfaces, and shapes in spaces.
The space in the image below illustrates the idea of degree of enclosure. Walls or the absence of them, and the use of columns, arches, and window openings create a unique form and space relationship. The degree of enclosure is key to the success of this space.
As it is an outdoor space adjacent to a house, the degree of enclosure in probably mid-range. While in the space, the users would feel halfway inside and halfway outside which is ideal as a transitional space. This is illustrated in the diagram below that shows the space and forms from above.
I hope that clarifies how form and space work together!