First of all, the content expressed is different:
What: when used as a pronoun, it means "what; How; When used as an adjective, it means "what; How; "How", when used as an adverb, means "to what extent and in what way", and when used as an exclamation, means "what; How about it? "
That: that, that, that person, that thing, that thing.
Second, the role is different:
What can be used as pronouns, adjectives, adverbs and interjections. Widely used in English, it can be used as adjectives, pronouns, conjunctions, adverbs and fixed collocations.
Third, the way to guide the terms is different:
When what leads a noun clause, it acts as a component of the sentence, such as subject, predicative, object or attribute.
The difference is that what means something in a general sense, which is often translated as "what" or "what", and whatever is its powerful language potential "whatever".
Can be used as a conjunction:
He expressed the hope that we would keep in touch with his company.
He expressed the hope that we could keep in touch with his company.
We are glad that we have completed another arduous task.
We are all happy because we have completed another arduous task.
He is studying hard in order to catch up with his classmates.
He studies hard in order to catch up with his classmates.
The main clause is often omitted: I can stay at home today! ?
I wish I could stay at home today!
Extended data:
What are the precautions for use:
1, "What ... is ..." is an emphasis sentence pattern, which is usually translated as "What is ...". What is usually used as the object of the subject clause, and sometimes it can also be used as the subject of the subject clause.
What bothers me most is that I don't know how to tell her the bad news. What bothers me most is that I don't know how to tell her the bad news.
2. "Is" can be followed by a general predicative or predicative clause. (For example, the above two examples)
I don't understand what you said. I don't understand what you said. (leading object clause)
The question is what we can tell him. The question is what we can tell him. (Leading predicative clause)
In addition, what can be used before infinitive, together with infinitive, as an object, subject or predication in a sentence.
For example:
I don't know what to do. I don't know what to do.
What to do has not been decided.
The question is what to do. The question is what to do?
References:
Baidu Encyclopedia-What
Baidu encyclopedia-that