A nice dynamic method of composing a photo is to have a diagonal line running through the photo, more or less from corner to corner. The line could be a street, a fence rail, a road or a shoreline. Anything which creates a line or division in the picture.
Whether the line rises or falls creates different feelings about the photo. Typically, for English speaking people and others with "left to right" direction of writing, if an line descends into the picture from top left to bottom right it appears to be entering the picture. If the line rises from bottom left to top right, it appears to be leaving the picture.
You can use those concepts to create specific feelings in your picture. Such as, a person standing beside a lake shore - if the line descends, it would suggest the person is entering the picture and thus would be, perhaps, happier and more inclusive. If the line is rising it would suggest the person is leaving and is thus more distant and removed.
Diagonal lines in photos can be used for what is called a "leading line" which helps the view be lead through the image in directed manner. Typically you'd have the lines "pointing" at the main subject of your photo - which puts the leading lines into the Negative Space (you'll come across more about Negative Space in Lesson 11.)
Exercise: Find a location where you can use a line to run diagonally through a picture. Take two pictures, one with the line descending into the picture from the left, and the other rising to right.