diff --git a/tools/verifygitlog.py b/tools/verifygitlog.py
index 5d45e4a4cc61..c237cb46beef 100755
--- a/tools/verifygitlog.py
+++ b/tools/verifygitlog.py
@@ -46,6 +46,23 @@ def git_log(pretty_format, *args):
         yield line.decode().rstrip("\r\n")
 
 
+def diagnose_subject_line(subject_line, subject_line_format, err):
+    err.error("Subject line: " + subject_line)
+    if not subject_line.endswith("."):
+        err.error('* should end with "."')
+    if not re.match(r"^[^!]+: ", subject_line):
+        err.error('* should start with "path: "')
+    if re.match(r"^[^!]+: *$", subject_line):
+        err.error("* should contain a subject after the path.")
+    m = re.match(r"^[^!]+: ([a-z][^ ]*)", subject_line)
+    if m:
+        err.error('* first word of subject ("{}") should be capitalised.'.format(m.group(1)))
+    if re.match(r"^[^!]+: [^ ]+$", subject_line):
+        err.error("* subject should contain more than one word.")
+    err.error("* should match: " + repr(subject_line_format))
+    err.error('* Example: "py/runtime: Add support for foo to bar."')
+
+
 def verify(sha, err):
     verbose("verify", sha)
     err.prefix = "commit " + sha + ": "
@@ -75,9 +92,9 @@ def verify_message_body(raw_body, err):
     very_verbose("subject_line", subject_line)
     subject_line_format = r"^[^!]+: [A-Z]+.+ .+\.$"
     if not re.match(subject_line_format, subject_line):
-        err.error("Subject line should match " + repr(subject_line_format) + ": " + subject_line)
+        diagnose_subject_line(subject_line, subject_line_format, err)
     if len(subject_line) >= 73:
-        err.error("Subject line should be 72 or less characters: " + subject_line)
+        err.error("Subject line should be 72 or fewer characters: " + subject_line)
 
     # Second one divides subject and body.
     if len(raw_body) > 1 and raw_body[1]:
@@ -90,7 +107,7 @@ def verify_message_body(raw_body, err):
             err.error("Message lines should be 75 or less characters: " + line)
 
     if not raw_body[-1].startswith("Signed-off-by: ") or "@" not in raw_body[-1]:
-        err.warning("Message should be signed-off")
+        err.warning('Message should be signed-off. Use "git commit -s".')
 
 
 def run(args):