쓰다 means "to write, to use, to employ, to be bitter (in taste)" 말 means "words, language, tongue (referred in language skill) or speech." 하다 means to do. So...To speak would be "말하다." literally meaning "To do speech, to do language, etc."

Well, I'm not a native speaker, but I'll give it a go! Did you mean "뭐하는 거야?" That means "What are you doing?" Firstly, '거야' is in the informal speech, so you probably shouldn't use this with someone older or of a higher title than you. Secondly, in this context of "뭐하는 거야?", the 는 거야 is used to indicate present tense. For example, "뭐하는 거야?"- What are you doing? "공부하는 거야."- I'm studying. You can also use 거야 when referring to the future tense. However, you must conjugate the root verb. For example: 하다- to do. " 뭐 할 거야?"- What are you going to do? If there is no consonant ending the word, you use 'ㄹ 거야' If the word DOES end in a consonant, you use "을 거야." (for example: 먹을 거야) The formal form of '거야." is "거예요." For example: "선생님, 오늘 뭐 할 거예요?"- Teacher, what are we going to do? I hope I helped you! If you're not satisfied with my answer, then message me and I will ask one of my native speaker friends to help you!