V + -sa/-se is called "dilek-şart" (wish and condition) that is it may be used to make conditional (hypothetical) statement or a wish
gelse = I wish s/he came (wish)
gelse = if s/he came (but s/he didn´t(doesn´t) (hypothetical condition)
Now in old Turkish we had a verb called "er-mek" (to be) and we had "er-se" form using -se suffix mentioned above.
Over time, "er-mek" became "i-mek" (with it various forms er-di > i-di; er-miş > i-miş; er-se > i-se; er-ken > i-ken; in Uyghur/Uzbek language: er-mez > i-mez = not). Then they all became suffixes:
i-se > -se/-sa
i-di > -(y)di/-(y)dı/-(y)du/-(y)dü/-(y)dı/-ti/-tı/-tu/-tü
i-miş > -(y)miş/-(y)mış/-(y)muş/-(y)müş
i-ken > -(y)ken
geliyor = s/he is coming
geliyor ise = geliyor-sa = if s/he is coming
gelir = s/he comes
gelir ise = gelir-se = if s/he comes
gelecek = s/he will come = s/he has an intention to come
gelecek ise = gelecek-se = if s/he will come;if s/he has an intention to come
geldi = s/he has come
geldi ise = geldi-yse = if s/he has come
or
geldi = s/he came
geldi ise = geldi-yse = if s/he had come (past conditional, non-hypothetical)
gel-se = if s/he came (hypothetical)
gel-se idi = if s/he had come (past hypothetical)
Example:
Past hypothetical condition example:
3 dakika önce gelse idi, Ali burada idi. = If he had come 3 minutes earlier, Ali was still here (But he didn´t come and he and Ali couldn´t meet)
Present tense hypothetical condition example:
3 dakika önce gelse, Ali burada olur. = If he came 3 minutes earlier, Ali would be here.