لما بلغ عشرين عاما = عندما بلغ عشرين عاما is correct .
لما ينصر = عندما ينصر
it's like (when) in english accompanied with an action
for example : when i read , i understand
ex 2 : are we going ? .. no .. we will go when he tells as .
about لما = ليسَ بعد is not correct ( at least for me , i don't know )
there is many لما depending on the Tashkeel , here we are talking about لَماَ Lamma,
but لِما Lima is another meaning ( لِما = why ? ).
if something is not correct , tell me :D