First you use a full stop, then you hold down alt and press the letter H and put in another full stop. .?.
If you are trying to insert the therefore symbol into a WORD DOCUMENT
Hold down the ALT key and type 8756
Hope the answer ur question Regards Al~Hash.
From System Preferences, turn on the "Show Keyboard & Character Viewer in menu bar" setting.
Then, the "Character Viewer" menu will pop up a tool that will let you search for any unicode character (by name) and insert it ? you're all set.
If using WORD for mac enable 'use maths autocorrect rules outside maths regions' Type \therefore
Source: Stackoverflow.com