/* The paragraph is the first descendent of its parent. */
p:first-child { color: blue; }

/* Paragraphs that are the 4th children. */
p:nth-child(4) { color: purple; }

/* Last one. */
p:last-child { color: fuchsia; }

/* Only one. */
p:only-child { color: red; }

/* You can invert these, too. */
div:not(:first-child) { background-color: lightgray; }