Straight Stop Valve (1/2-in FIP * 3/8-in Comp, 2)

I shutoff main shutoff valve that is outside but water keeps running from sink and pipes would this make teflon not work because getting wet before?

I installed 1/2" npt male thread brass pipe nipple adapter to the 1/2 inch side valve and added teflon but is leaking slowly. does it need a washer?

The 1/2 inch side of this shutoff valve when used on a threaded pipe will it leak without teflon or teflon is required so it doesn't leak?