How do Placement Games Work?

Is it possible at all to place higher than you were last season through early placements? I just don't understand. I only play duo queue now because last season I placed silver for the first time and dropped to Bronze II. Playing in solo queue and duo queue, I got to my promos eight times last season but lost every time due to bad teams and comps. I stopped playing by myself in because of that and only played with my friends who are higher ranks so I should have decent MMR, right? I won 8 out 10 of my placements and carried half those games and yet I got placed even lower than before (Bronze IV). I want to escape bronze, but I need to know how MMR works and placements first I guess.