Add an S to the end of the word and it becomes more then one. Add two S’s and you will end up with one. What is the word?

I’m what you need to understand, but I’m not something you can simply hand. I’m in a joke, a tale, a text, a debate. Remove me, and your words lose sense.

I plummet low but do not break, and rise but never fly While I myself cannot be seen, it’s my effects you spy With several ways to measure but in truth there’s mainly two I only seem to disappear when I’m the same as you

We check on it when buying used things. We fight for it when we negotiate. We can set it. We often accept it without investigating.

I speak the truth by proving it false, and reveal a lie by making it true. What am I?