Spelling and Grammar
You can use the gear menu->Proofing submenu to start checking spelling ⌘+;, and opt to also check for grammar suggestions ⇧+⌘+:.
To navigate errors, use ⌥+⌘+; (next) and ⌥+⇧+⌘+: (previous).
Click the marker on the left to see details about the error, including suggestions for fixing.