Sadly, we do need to know the roots. From Lesson 4 onwards, you will find some of the tenses/types of verbs are based on the root (eg. aorist, which is introduced in that lesson) and others (eg. imperative), are based on the “present stem”, which is what we learnt in Lesson 1.
If you don’t know the roots, you are forced to learn each word multiple times (in all the different tenses and conjugations). You also need to learn to recognise the verb in its many conjugations, and not knowing the root will make it that much harder, since all the conjugations are ultimately derived from the root.
Knowing the root also gives you the ability to sometimes “guess” or infer the meaning of new words based on the same root but a different prefix (although this sometimes doesn’t work). For example, vadati means “speak” and abhivadeti means “greet or salute” (both based on the root vad). Another example: veṭheti means wrap and nibbeṭheti (both based on veṭh) means “untwist” or “unravel”. There’s a whole bunch of words based on pad and kam but with different prefixes that are kind of related to each other.
Warner also gets lazy in the latter chapters and gives words in a non present tense (eg. bhinna in Lesson 11 page 64 which means “divided, split”) but he doesn’t give you the present equivalent, so you are left to figure out how to construct the present from the root which is bhin. And to do that, you need to know the conjugation group so you can apply the right rules (Warder doesn’t tell us). Hence my question.