Replaced "can not" with "cannot" and "could not" in a bunch of places.
Fixed nearby grammar errors.

src/http/modules/ngx_http_dav_module.c

diff --git a/src/http/modules/ngx_http_dav_module.c b/src/http/modules/ngx_http_dav_module.c
index 0761c16..3bd9d3e 100644
--- a/src/http/modules/ngx_http_dav_module.c
+++ b/src/http/modules/ngx_http_dav_module.c
@@ -158,7 +158,7 @@
 
         if (r->uri.data[r->uri.len - 1] == '/') {
             ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
-                          "can not PUT to a collection");
+                          "cannot PUT to a collection");
             return NGX_HTTP_CONFLICT;
         }
 

src/http/modules/ngx_http_headers_filter_module.c

diff --git a/src/http/modules/ngx_http_headers_filter_module.c b/src/http/modules/ngx_http_headers_filter_module.c
index 1d40958..c3f74ae 100644
--- a/src/http/modules/ngx_http_headers_filter_module.c
+++ b/src/http/modules/ngx_http_headers_filter_module.c
@@ -507,7 +507,7 @@
         minus = 0;
 
         if (hcf->expires == NGX_HTTP_EXPIRES_MODIFIED) {
-            return "daily time can not be used with \"modified\" parameter";
+            return "daily time cannot be used with \"modified\" parameter";
         }
 
         hcf->expires = NGX_HTTP_EXPIRES_DAILY;

src/http/modules/ngx_http_log_module.c

diff --git a/src/http/modules/ngx_http_log_module.c b/src/http/modules/ngx_http_log_module.c
index b2bfbf4..9728d54 100644
--- a/src/http/modules/ngx_http_log_module.c
+++ b/src/http/modules/ngx_http_log_module.c
@@ -960,7 +960,7 @@
 
         if (log->script) {
             ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
-                               "buffered logs can not have variables in name");
+                               "buffered logs cannot have variables in name");
             return NGX_CONF_ERROR;
         }
 

src/http/ngx_http.c

diff --git a/src/http/ngx_http.c b/src/http/ngx_http.c
index 5ca9fed..e809045 100644
--- a/src/http/ngx_http.c
+++ b/src/http/ngx_http.c
@@ -1226,7 +1226,7 @@
 #endif
 
     /*
-     * we can not compare whole sockaddr struct's as kernel
+     * we cannot compare whole sockaddr struct's as kernel
      * may fill some fields in inherited sockaddr struct's
      */
 

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 0fb5012..d51dc23 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -3191,7 +3191,7 @@
                               prev->underscores_in_headers, 0);
 
     if (conf->server_names.nelts == 0) {
-        /* the array has 4 empty preallocated elements, so push can not fail */
+        /* the array has 4 empty preallocated elements, so push cannot fail */
         sn = ngx_array_push(&conf->server_names);
 #if (NGX_PCRE)
         sn->regex = NULL;