The word "that" can play the role of an adverb, conjunction or pronoun:
"What" with the adverb function
In some sentences, the “what” can play the role of an adverb of intensity or mood.
1. Adverb of intensity: What a big fish the fisherman caught! (The fisherman caught a very big fish.)
2. Mode adverb: What weird people! (How weird these people are!)
"What" with the conjunction function
There are situations whose “what” plays the role of conjunction of cause, comparison, concession, consequence, explanation, purpose, integration and time.
1. Causal conjunction: Now I'm going to lie down, I'm tired. (It expresses the cause of my wanting to lie down: tiredness. It's like saying “I'm going to lie down now, because I'm tired.”)
2. Comparative conjunction: This place is nicer than that. (Makes a comparison between one place and another.)
3. Conjunction concessive: Even if he doesn't, I will. (It expresses the opposite idea, but that does not prevent the action, that is, not being allowed to leave, but to leave.)
4. Consecutive conjunction: Ate so much that he was sick. (It manifests a consequence, namely, the fact of having eaten so much resulted in a feeling of unease.)
5. Explanatory conjunction: I'm going to leave a little that is good for my head. (Indicates a justification. It's like saying “I'm going out for a bit, it's good for my head.)
6. Final conjunction: I left without giving an answer so the fight wouldn't escalate. (It expresses the purpose, that is, to leave without giving an answer in order and the fight does not increase.)
7. Integral conjunction: I want you to be happy. (They introduce substantive subordinate clauses.)
8. Temporal conjunction: As soon as I'm done, let's leave. (Express a circumstance of time.)
"What" with the pronoun function
The “what”, in many situations, plays the role of a relative or interrogative pronoun.
1. Relative Pronoun: I bought the books on the material list. (The relative pronoun "that" relates to the term "books". Notice how the prayers would be separate without it: The books are on the material list. I bought the books.)
2. Interrogative pronoun: What is this place? (The interrogative pronoun “that” is used in interrogative clauses.)
For you to know more: What or what?