Some questions already asked on this topic, but l wonder how to prevent water entering deeper than nostrils while underwater (ie : to lungs), especially when you look at the surface of the water? l can hold my breath underwater, even breath out, but in that case l can not hold my breath not more than 20 seconds ...
If a technique exists, thanks !

If you have large enough lips you can curl your upper lip until it covers your nostril. It almost looks like you're trying to kiss someone and looks really funny but it works. The other option is a nose plug.

when l was little, l took swimming lessons, but then l got water in my nose. all hell broke loose! my swim instructor told me to blow bubbles out my nose, and l did that, and it worked great. Now l swim competitively, and if you dont think about ur nose, nothing happens.
good luck! :)

slowly let water out your nose and mouth. a nose plug works good too, the kind that pinch your nose towards the top.

Breathe out through your nose. Just let out small amounts of air so that the water wo not go into your nostrils, and so that you can stay under a long time.