"What" is more of a question word. "That" is used to describe or define something. Examples: "What are you doing?" "What is this?" "I think that this is blue." "I do not like that dress." I hope this helped a bit :)

That is wrong. You can use that at the beginning of a sentence - I just did! That (usually) indicates an particular object or statement, whereas what is used to enquire about one. That also can be used following clauses like 'I think...', 'I feel...', 'I hope...'

Since you specifically said that you were not asking about the use of "what" in questions I think it's safe to assume you are asking about the use of "what" and "that" as relative pronouns. I could give a very long answer about relative pronouns in general but in the interests of simplicity I will give you two basic rules about the uses of "what" and "that". "What" refers to things, qualities, or ideas, never to people or animals. For example, "I know what happened to your bicycle" or, "The expert knew what he was talking about". "That" refers to people, animals, things, ideas and qualities. For example, "The woman that witnessed the crime died two days later" or, "The man found the cave that contained the bear". If you are still confused about this I'd be happy to talk about it next time you are on Skype. It's actually quite a complex subject so I can imagine you might need some extra help :) Hasta luego :)

Carljul, did you notice that you used the word "what" in your own question? My answer: One of them is most OFTEN used at the beginning of a question! One of them is NEVER used at the beginning of a question. :) Another example: "WHAT is your name?" (My name is Sandy.) "Oh, I see the word SANDY near your picture! So, THAT is your name!"