when we are using DO as auxillary verb the main verb used should be in which form, base form or V1? Example :I do understand. Here 'Understand' is in Base form or V1?
After any part of do (do, does, did, done, don't, doesn't, didn't) the verb it governs is always in the base form.
Edit: I recognise "V1" from questions here as terminology which is used in TESOL, but it is unknown to most English speakers who are not TESOL teachers, and I didn't know what it meant without looking it up: it appears to mean the base form, so I don't understand (!) your question.