what's the difference between vIlaDtaH and vilaD?
The aspect suffix -taH indicates that an action is (or was or will be) on-going.
In this course, we translate it into English with an -ing form (progressive, continuous), so vIlaDtaH is "I was reading, I am reading, I will be reading".
Klingon verb forms without an aspect suffix are translated on this course into simple forms in English: vIlaD is "I read (pronounced "red"), I read (pronounced "reed"), I will read". (simple past, simple present, simple future)
vilaD isn't a Klingon word; Klingon doesn't have the (lowercase) letter i.