Clauses come in four types: main [or independent], subordinate [or dependent], adjective [or relative], and noun. Every clause has at least a subject and a verb. Other characteristics will help you distinguish one type of clause from another.
My native language is not English as well, but I'm sure these sentences have different meanings. Par exemple, "I speak as I eat - slowly". Or "I speak when I eat (and only when I eat)". So "I speak WHILE I eat" describes the exact situation when you speak and eat at the same time.