"No, tú no caminas."
Translation:No, you are not walking.
The present progressive is used only when one (or others) are in the mist of doing something at that moment, right then. Other wise present tense MUST be used in Spanish. English does not translate to Spanish word for word. This a funny sentence anyway. We English speakers can say things in various ways and mean the same way.
"Son" is used in a sentence saying what people are, with "ellos" and "ellas." For example: "Ellos son hombres" means "They are men."
"They are walking" would be "Ellos caminan," not "Ellos son caminan." The reason for this is that "They are walking" is not the same as "They are men." See what I mean?
Anyways, this sentence deals with "Tú," not "Ellos." I hope this helps you!