diff --git a/How-to-issue-a-cert.md b/How-to-issue-a-cert.md
index 2acaddb..31ecded 100644
--- a/How-to-issue-a-cert.md
+++ b/How-to-issue-a-cert.md
@@ -1,7 +1,10 @@
+## Challenge types
+First, see [Challenge Types](https://letsencrypt.org/docs/challenge-types/) to understand the difference between methods of domain validation over HTTP, DNS or TLS ALPN.
+
 ### 1. Single domain:
 
 #### 1) Webroot mode:
-If you already have a web server running, you should use webroot mode.  you only need write access to the web root folder.
+If you already have a web server running, you should use webroot mode. You only need write access to the web root folder.
 
 ```sh
 acme.sh  --issue  -d example.com  -w /home/wwwroot/example.com
@@ -9,7 +12,7 @@ acme.sh  --issue  -d example.com  -w /home/wwwroot/example.com
 
 #### 2) Standalone mode:
 
-If you don't have a web server,  maybe you are on a smtp or ftp server, the 80 port is free. you can use standalone mode.
+If you don't have a web server, maybe you are on a SMPT or FTP server, the 80 port is free. You can use the standalone mode.
 acme.sh has a builtin standalone webserver, it can listen at 80 port to issue the cert.
 
 ```sh
@@ -43,13 +46,11 @@ Yes, if your nameservice provider has an api, we can use the api to automaticall
 Cloudflare api:
 ```sh
 export CF_Token="sdfsdfsdfljlbjkljlkjsdfoiwje"
-
 export CF_Email="xxxx@sss.com"
-
 acme.sh  --issue  -d example.com  --dns dns_cf
 ```
 
-How to use dns api:  https://github.com/acmesh-official/acme.sh/wiki/dnsapi
+How to use dns api: https://github.com/acmesh-official/acme.sh/wiki/dnsapi
 
 #### 5) DNS manual mode:
 
@@ -71,19 +72,19 @@ acme.sh  --issue  -d example.com  --apache
 
 If your website is running nginx server, acme.sh can use nginx server to issue cert. And acme.sh will restore your nginx conf after the cert is issued,  don't worry.
 
-```
+```sh
 acme.sh  --issue  -d example.com  --nginx
 ```
 
-Sometimes, nginx conf file can not be found automatically, you can specify one:
+Sometimes, Nginx conf file can not be found automatically, you can specify one:
 
-```
+```sh
 acme.sh  --issue  -d example.com  --nginx /etc/nginx/nginx.conf
 ```
 
 You can also specify the website conf:
 
-```
+```sh
 acme.sh  --issue  -d example.com  --nginx /etc/nginx/conf.d/example.com.conf
 ```
 
@@ -94,27 +95,25 @@ Issue a single cert including multiple domains.  All the domains use the same va
 
 #### 1) Webroot mode:
 You must point `example.com` and `www.example.com` to the same web root folder `/home/wwwroot/example.com`
-```
+```sh
 acme.sh  --issue  -d example.com  -w /home/wwwroot/example.com   -d www.example.com
 ```
 
 #### 2) Standalone mode:
-```
+```sh
 acme.sh  --issue  -d example.com  --standalone  -d www.example.com 
 ```
 
 #### 3) Dns api mode:
-Cloud flare api:
-```
+Cloudflare API:
+```sh
 export CF_Token="sdfsdfsdfljlbjkljlkjsdfoiwje"
-
 export CF_Email="xxxx@example.com"
-
 acme.sh  --issue  -d example.com  --dns dns_cf  -d www.example.com
 ```
 
 #### 4) Dns manual mode:
-```
+```sh
 acme.sh  --issue  -d example.com  --dns  -d www.example.com
 ```
 
@@ -123,7 +122,7 @@ acme.sh  --issue  -d example.com  --dns  -d www.example.com
 Issue a single cert including multiple domains.  Each domain uses a different validation method.
 
 
-```
+```sh
 acme.sh  --issue  \
 -d aa.com  -w /home/wwwroot/aa.com \
 -d bb.com  --dns dns_cf \
@@ -131,10 +130,9 @@ acme.sh  --issue  \
 -d dd.com  -w /home/wwwroot/dd.com
 ```
 
-```
+```sh
 acme.sh  --issue  \
 -d aa.com  --dns dns_dp \
 -d bb.com  --dns dns_cf \
 -d cc.com  --dns dns_ns 
 ```
-