A full synopsis should be, as mentioned, third person present tense, clear and simple, plainly factual. Don't hide anything, tell the whole story; secrets, surprises, reveals and all.
Now there are some who would say, don't give it all away in the synopsis. But then what you are giving them is just a taster. The purpose of a full synopsis is to demonstrate to the publisher/agent that you have the whole story plotted out and that it all hangs together without them having to read the whole MS.