could you clarify the difference between formal and informal language?
Formal language stays within officially recognized rules. For instance, here in the U.S., an organization called Modern Language Association decides what is correct and incorrect.
Informal language includes colloquialisms, regionalisms, slang, and alternate structures and grammars. Double negatives, for instance, are common in informal English, but not permitted in formal English.
could you clarify the difference between Written and Spoken language?
Written English follows all the rules of spoken English, but has several other groups of rules that have to do with such things as spelling, punctuation, and capitalization. Obviously, those issues don't exist in spoken language.