yes, both are simple present (second incorrect) 1st person singular (I) and plural (We), 2nd person singular (You) and plural (You), 3rd person plural (They) + V 3rd person singular (He, She, It) +Vs

it depends on te situation the first sentence "you walk very fast" means that he allways walk fast as you used the present simple in ,by using this sentence you will not give the meaning that he is walking very fast at the moment you are talking to him in because its general. the second sentence is incorect but if you want you may say "you are walking very fast" thus it means that the other person you are talking to is walking very fast at the moment you are talking to him at and we use the present continous to give this meaning by adding "ing" to the verb

"You walk." It is simple present tense. I / you / we / they walk. (no -s) He/ she / it walks. (add -s)

"You walk very fast" is grammatically correct It is in present tense

Yes, it is Simple Present Tense. Q1: It is the right one. Because of the simple fact of its rules. Check this useful site. http://www.ego4u.com/en/cram-up/grammar/simple-present/form