The base form is 멈춰서다 (to stop (in one's track)). 

It is a compound verb comprising 멈추다(stop) and 서다(come to a stop) but it behaves just like any other verb.

1. 멈춰서도 = 멈춰서(다) + -도 (verb ending expressing "even if (you do the action)") => even if I stop (not a full sentence).

2. 멈춰섰다 = 멈춰서(다) + -(아/어)ㅆ다 (sentence ending form in the past tense) => (I) stopped (full sentence).

There is a whole bunch of verb forms (conjugations) besides '-도',  and another set of sentence endings.  Knowing these forms is probably the most important part of learning Korean.