Shapes, darks, and colors are the building blocks of any art, be it a painting, a sketch, or a photograph. Of the three, color is the least critical, however, shapes and the relationship between dark and light are the most important – black and white photographs use only shapes and darks often with striking results.
Shapes are the foundation for sketching as our mind quickly recognizes shapes, even with little detail. We can suggest shapes with lines or with paint.
Though there is little detail in either of these sketches …our mind identifies the shapes and both would qualify as a genuine sketch. Frequently when traveling with little time to stop and sketch you can capture a moment in five minutes or less. Our mind has a catalogue of images, we think in pictures, that is why it works.
Getting the shapes right does not mean precise. Loose approximations are often the best sketches, a style favored by many. There just has to be enough clarity to trigger the mind to fill in the blanks.
We also interpret the shapes based on their relationships to each other. In the line drawing the people are larger than the house, yet, because of the placement of each, our mind knows that the house is further away. In reality the Eiffel Tower is 180 times taller than a person, yet in the blue painting it all makes sense.
Here is a five minute sketch I did of Yosemite Falls, standing, with little time to sketch. The color was added later, but it was an identifiable sketch without the color because of the shapes.

Darks, for our purposes include shade, shadows, and dark vs. light colors. The rule is this – Get the darks right and the rest is easy. More on this later, when we get to discussing color.
Just a bit of shadow and shading makes a big difference to our simple sketch. Notice that the color I used for the shading does not matter, most any color would have worked, though blues or grays work best.

Before you begin to draw or paint take a moment to identify the major shapes, ignore the details. In this scene we have mountains, a house, hills, a tree, and people.
Next pick out the darkest areas. One trick that artists have used for many years is to squint, the dark areas will stand out, remember them. Some look at the scene through a red filter, it takes out all of the color and makes the darks easy to see.
After identifying the darkest areas next find the lightest areas, squinting or a red filter works for this as well. What is left after the lights and the darks are the mid-tones.
Another way is to take a photo then turn it into a black and white.
Notice how the dark areas stand out. The lightest areas are the sky, the edges of the buildings, the side of the building on the left. The rest are the mid-tones. If you get the darks and the lights right, the mid-tones can be off, yet the sketch will be quite impressive.