diff --git a/src/edu/mills/cs180a/StringUtilities.java b/src/edu/mills/cs180a/StringUtilities.java
index 31f03b8..7d10a83 100644
--- a/src/edu/mills/cs180a/StringUtilities.java
+++ b/src/edu/mills/cs180a/StringUtilities.java
@@ -40,12 +40,19 @@ public static boolean isSubstring(String substring, String text) {
 
   // check if substring appears at the given offset in text
   private static boolean isSubstringHelper(String substring, String text, int offset) {
+    // Return false if the substring is longer than the part of the text after
+    // the beginning of the substring
+    if (substring.length() - 1 > text.substring(offset).length()) {
+      return false;
+    }
+
     // i is used as an index for substring, offset is used for text
     for (int i = 1; // The character with index 0 has already been tested.
         i < substring.length() && offset < text.length(); i++, offset++) {
       if (text.charAt(offset) != substring.charAt(i))
         return false;
     }
+
     return true;
   }
 
diff --git a/src/edu/mills/cs180a/StringUtilitiesTester.java b/src/edu/mills/cs180a/StringUtilitiesTester.java
index a38617b..a9eba67 100644
--- a/src/edu/mills/cs180a/StringUtilitiesTester.java
+++ b/src/edu/mills/cs180a/StringUtilitiesTester.java
@@ -1,8 +1,11 @@
 package edu.mills.cs180a;
 
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 class StringUtilitiesTester {
   @Test
@@ -13,13 +16,86 @@ void isSubstring_False_null() {
   }
 
   @Test
-  void isSubstring_False_EmptyString() {
+  void isSubstring_True_EmptyString() {
     assertTrue(StringUtilities.isSubstring("", "foo"));
-    // TODO: write rest
   }
 
   @Test
   void isSubstring_True_Length1Substring() {
     assertTrue(StringUtilities.isSubstring("A", "ABC"));
   }
+
+  @Test
+  void isSubstring_True_Length2Substring() {
+    assertTrue(StringUtilities.isSubstring("C D", "ABC DEF"));
+  }
+
+  @Test
+  void isSubstring_True_Length3Substring() {
+    assertTrue(StringUtilities.isSubstring("GHI", "ABC DEF GHI"));
+  }
+
+  @Test
+  void isSubstring_False_Length1Substring() {
+    assertFalse(StringUtilities.isSubstring("D", "ABC"));
+  }
+
+  @Test
+  void isSubstring_False_Length2Substring() {
+    assertFalse(StringUtilities.isSubstring("C F", "ABC DEF"));
+  }
+
+  @Test
+  void isSubstring_False_Length3Substring() {
+    assertFalse(StringUtilities.isSubstring("XYZ ABC", "ABC DEF GHI"));
+  }
+
+  @Test
+  void isSubstring_True_SubstringAtStart() {
+    assertTrue(StringUtilities.isSubstring("ABC", "ABC DEF GHI"));
+  }
+
+  @Test
+  void isSubstring_True_SubstringInMiddle() {
+    assertTrue(StringUtilities.isSubstring("DEF", "ABC DEF GHI"));
+  }
+
+  @Test
+  void isSubstring_True_SubstringAtEnd() {
+    assertTrue(StringUtilities.isSubstring("GHI", "ABC DEF GHI"));
+  }
+
+  @Test
+  void isSubstring_False_SubstringAtStart() {
+    assertFalse(StringUtilities.isSubstring("ADF", "ABC DEF GHI"));
+  }
+
+  @Test
+  void isSubstring_False_SubstringInMiddle() {
+    assertFalse(StringUtilities.isSubstring("C DEG", "ABC DEF GHI"));
+  }
+
+  @Test
+  void isSubstring_False_SubstringAtEnd() {
+    assertFalse(StringUtilities.isSubstring("Ijk", "GHI"));
+  }
+
+  @ParameterizedTest
+  @CsvSource({"The cart, tottenham", "Tra lalalalala,apple pie", "Mo Ghile Mear,run away"})
+  void isSubstring_Fail_FirstLetterOfSubstringLastLetterOfString(String input, String subInput) {
+    assertFalse(StringUtilities.isSubstring(subInput, input));
+  }
+
+  @ParameterizedTest
+  @CsvSource({"The cat is in the hat,cat is in", "Tra lalalalala,Tra", "Mo Ghile Mear,Ghile"})
+  void isSubstring_True_VariedStringsAndSubstrings(String input, String subInput) {
+    assertTrue(StringUtilities.isSubstring(subInput, input));
+  }
+
+  @ParameterizedTest
+  @CsvSource({"The cat is in the hat,hotpot", "Tra lalalalala,homeward is",
+      "Mo Ghile Mear,run away"})
+  void isSubstring_False_VariedStringsAndSubstrings(String input, String subInput) {
+    assertFalse(StringUtilities.isSubstring(subInput, input));
+  }
 }