My 2yr old has a large white, hard spot on front left corner or tongue. Is it burn, hard bite or disease? He seems happy but fussy eating.

Not burn. It does not sound like a burn...It is difficult to tell from your description. An aphthous ulcer (not dangerous) can appear white, but not hard. This is usually painful and so the child can be fussy trying to eat. Have the kid checked out if it is more than a few days...
Tongue. A burn is probably not as likely. Most 2 year olds may not be eating very hot foods. A bite is possible. Disease is also probably not as likely. However, i would suggest you have your pediatrician have a look and refer to derm or peds ENT as necessary.