Japanese plan for Midway was to lure Americans there by surprise. Of course we knew what was up but..... one of the Japanese plans to prevent their surprise was a line of submarines as spotters, they just got there too late as the carriers were long gone what if one of the subs got there a little early and spotted Yorktown (which departed later than main body) got spotted how significantly would this have changed the outcome, how would this outcome have changed the course of the war

Well thats a interesting one.

I think what may have happened is that the Battle for Midway would have done one of two things:
1 would have been it would have lasted alot longer since the Japs would have had better intelligence on us and both sides would have had waaayy more inflicting casualties.
2 would be the Japs would have won.

Either way I dont think it would have allowed the Japs to effect the war significantly since we were going to drop the A-bomb instead of invading the homeland.

Yea, I think both sides would have gotten clobbered in this scenario the short term effect probably would have been a delay in the Marine island hopping campaign..specif: Guadalcanal, but with the Essex class already under construction the war would have progressed pretty much the same with just longer leaps on island hopping, Hell we might have bypassed that screw up at Peleilu

I'm not all that sure it would have had any effect at all. The Japanese were suffering badly from 'victory disease' at this time. If they had received the report they might very well have said "Oh good, another victim for our trap." Assuming a sub spotted the Yorktown there probably isn't any way they would have known it was her and not Enterprise or Hornet. They would only know that one carrier was in the vicinity, not where the other two were. This might have made Nagumo more cautious but that would probably have been about it.

Given Nagumo's caution, I think he would have had a strike force on standby. This probably would have had the effect of his carrers not being cluttered up with bombs and torpedoes laying everywhere. Possible our SBD hits wouldn't have been as catastrofic thus enabling a possible strike on the main body. Or possible not sinking all of the IJN ones.