The magic word for being able to sing long phrases is: Enunciation.
If you learn how to enunciate any consonant or consonant cluster without
pressing down heavily on the vocal mechanism you will have a fair
chance that the sound travels right underneath your palate.
If your sound travels that high you don't loose much air while you sing.
That is why some singers cannot walk up a flight of stairs without being completely out of breath but they can sing very long phrases. It is all about technique.
When you use your enunciation correctly you help your lunges.
They work just as a sponge does. When we squeeze a sponge and get it "dry" it then
will draw in the liquid very quickly. You don't have to do anything for it to soak itself full.
That is an automatic mechanism. All you have to do is squeeze it and release the pressure.
The same applies to our lunges. By singing correctly you will
help that mechanism that can fill your lunges with air in a fracture
of a second. If you try to pull the air in you are already on the wrong track.
You don't try to pull water into a sponge either. The sponge is designed
to soak up the water and so your lunges are designed to take in the air.
If you pull in the air by force you will get a breath that does not go deep
into the belly and therefore will not be in your control when you sing.
You will feel a lot of pressure (air pressure) against the chords and
after singing just a few words all this air will have been released.
But how do you learn to use your vocal mechanism correctly to avoid that
and to be able to use the air in small quantities. An even stream of air that
you can control. That will make your phrase long and give you a beautiful legato line.
The bad news is that it takes years of study and practice. However I can give you some hints
and exercises and you will have a positive effect if you do them over
a period of time. Don't expect any effect if you try it only once.
Think of your tongue as a very precise instrument. It should never press down onto the throat and
The tongue needs to know every position of every consonant.
By that I mean you need to know where your tongue touches the palate,
the sides of the palate or the teeth. Every consonant has a different position.
You want to avoid enunciating with a heavy tongue, or the back of the tongue,
pressing downwards onto the back of the throat. Instead think
that you "flick" away from the position you have held with a very
precise and small "flick". That "flick" releases away from the back
of the tongue as if it could "tickle" the palate to go up a little bit higher.
Use only as much muscle power as you really need.
A very good way to practice this is to think that you are a ventriloquist.
Speak the words in a way that your mouth and face muscles don't
show movement on the outside. This opens the throat and brings stillness
to the vocal apparatus. After some time your throat will stay still
and the adams apple will stay still. The larynx will lower and you can sing with more ease.
Enunciate as much as possible only with the tip of the tongue and be as precise as you can with the positions of the consonants.
Just by switching your focus onto the inside and not allowing your
outside muscles to show movement (ventriloquist again) you will have a chance for great improvement. The throat will likely open for you and your efforts will be met with much more results.
There is a lot to that subject and it is hard to describe it. It takes in depth study but at least you know more and maybe you can feel an improvement just by focusing on using the tip of the tongue and by singing like a ventriloquist would speak. Don't open the mouth so much - open the throat!