These two words are rightly considered the most confusing language units among the seemingly simple ones. I mean, do you see any difference between them?
"Certainly, I do" - that's what one would say. However, 60% of the answers are usually not true. A learner could possibly share the examples of sentences "how" and "what" are used in, but if he's asked to substitute one word with the other, will he start hesitating if the previous version of the sentence is correct?
There is a significant difference between "how" and "what". Let's make it clear.
Sometimes even a native speaker of English can not tell you, what parts of speech these words are. Well, they are both interrogative words. But here comes the first difference.
"What" is either an interrogative determiner or an interrogative pronoun. "How" is an interrogative pro-adverb. That means the former "replaces" nouns, and the latter stands for adverbs in interrogative sentences.
Examples: There's a notebook on the table. - What is on the table?
They ran away fast. - How did they run away?
However, both "how" and "what" are relative pronouns when used in relative clauses.
"OK, I've been told about that before and that's easy for me" - you say again. True, this all is not worth further consideration. But here are some examples of mistakes I want to prevent you from making.
1. How do you think?
If you ask me, well, I think with the help of my brain. As you have already understood, this question is never asked if one wants to learn his collocutor's opinion. Say "What do you think?".
But what if I shout out: "How do you think I am going to be perfect at English in two days?!" Paradoxically at first sight, this sentenceis correct. I have met at least 10 students who replaced "how" with "what" having read the previous paragraph.
Now let's take a look at what happens if do such a thing: "How do you think I am going to be perfect at English in two days?!" And now we turn it into a correct interrogative sentence: "Howam I going to be perfect....etc.?!"
Look at the initial sentence and note that the meaning is still the same, so "how" does not refer to the "you think" clause. Therefore, you may freely use it in such a case.
2. How to say...?
You should never start a question with this phrase. Simply because it doesn't make sense when standing there. A question like that always contains an auxiliary or modal verb after how, hence use:
"How do(es) you (one) say.... ?" (just choose compatible words)
3. How come you to New York?
"How come" means "why". The verb "to come" doesn't turn into a modal one, so that you can only laugh at a question titling the item.
The usage of "how come" does not require inversion (changing the order of words to make a question) within the sentence. Example:
"Why didn't you bring it with you?" - "How come you didn't bring it with you?"
4. Supp bro?
As mentioned above, "what" is an interrogative pronoun that substitutes anoun, therefore. Learners adore saying "What's up?", but, when asked, most of them answer "good", "fine" or "OK". These are answers for "How..."-questions, but here's "what".
So the right way to react is:"Nothing", "Not much", "Lots of stuff"or just"What's up?"
NOTE: Items 3 and 4 tell about the colloquial phrases.
P.S. "Howdy" is a Texas greeting, and a whatnot is a piece of furniture.