No hands are needed to break me, no box can hold me, and keeping me matters only after I have been tested. What am I?

I am first in end, last in place, and appear four times in experience. What am I?

A camera needs me before the picture is sharp; a meeting loses me when every phone starts buzzing. What am I?

At a border I may be crossed; in a queue I may be joined; in a script I may be delivered. Same word, three places. What am I?

I can stop a sale, protect a boundary, end a request, or save a life. I am only two letters. What word am I?

In a debate I may be made, missed, sharpened, or conceded, though I have no metal tip. What am I?

A violin string needs enough of me to sing; a room after an argument can have too much of me, even in silence. What am I?

A bank studies me before lending, a writer wants me after a quotation, and a card can run out of me at the worst moment. What am I?

A diver feels me in deep water, a student before an exam, and a tyre when it is inflated. Too much of me can crush. What am I?