diff --git a/c_modern_approach/Expressions/theory.c b/c_modern_approach/Expressions/theory.c
new file mode 100644
index 0000000..54f8634
--- /dev/null
+++ b/c_modern_approach/Expressions/theory.c
@@ -0,0 +1,59 @@
+#include <stdio.h>
+void undefined_subexpression() {
+  int b, c;
+  int a = 5;
+  c = (b = a + 2) - (a = 1);
+  printf("%d", c);
+}
+
+void comparison_unexpected() {
+  int i, j, k;
+  i = 10;
+  j = 2;
+  k = 0;
+  // This is equivalent of (i < j) < k because it is < is left-associated
+  // operator
+  printf("\n%d", i < j < k);
+}
+
+void switch_playground() {
+  int grade = 1;
+  switch (grade) {
+  /* case 0: */
+  /*   printf("Failint"); */
+  /*   break; */
+  /* case 1: */
+  /*   printf("Poor"); */
+  /*   break; */
+  /* case 2: */
+  /*   printf("Average"); */
+  /*   break; */
+  /* case 3: */
+  /*   printf("Good"); */
+  /*   break; */
+  /* case 4: */
+  /*   printf("Excelent"); */
+  /*   break; */
+  /* default: */
+  /*   printf("Illegal grade"); */
+  /*   break; */
+  /* } */
+  case 0: case 1: case 2: case 3: case 4:
+    printf("Allowed grades");
+    break;
+  default:
+    printf("Illegal grade");
+    break;
+  }
+}
+
+int main() {
+  int i = 5;
+  float k = 1.1;
+  float j = 10 * i + k;
+  printf("%f\n", j);
+  undefined_subexpression();
+  comparison_unexpected();
+  printf("\n");
+  switch_playground();
+}