Vali ipradis se nagesi issi
"Vali ipradis se nagesi issi."- The men are eating and sweating. Why is there a "se" in the sentence? I thought there was no "and" in High Valyrian or does that only imply to nouns?
Also when do you use issi/issa?
issa and issi are present indicative forms of the irregular verb sagon. issa is used for "he/she/it is", issi is used for "they are".
ipradis and nagesi are both verbs and have been conjugated correctly to use with a plural noun such as vali. There seems no reason to use issi in the sentence you have given.
If this was from the course, it is likely an erroneous recommended answer that crop up sometimes on Duolingo because of the confusion caused by the condensed way their sentence verifier works. But maybe it was from elsewhere.
At the moment it reads (to me) something like: "The men eat and they sweat are."
There could be legitimate compound verb forms which are more advanced than my level of study, which could make better sense of it, but for now I will believe the sentence is in error.
The tips and notes for the first skill of the course talk about vowel lengthening for simple coordination. The implication of the explanation (in my opinion) is that when the structure is more complex than a simple sequence of either nouns or adjectives or verbs, vowel lengthening is much less likely to be used; for those situations se exists.
Given what the tips and notes say about even verbs in the Simple Coordination section, it is probable that the se is unnecessary - or at least not the only way that the sentence could have been written, so long as the appropriate strategy for marking verb coordination (presumably vowel lengthening) was used. However, I do not recall seeing this strategy of coordination used for verbs within the course. (And whether there is a nuance to meaning is unclear to me.)
I would like to take a moment to state that I am unconnected with the course, and my comments are simply those of another Duolingo user (so, not official).