Two sounds from one spelling, and the wrong ones learners reach for
One pair of letters spells two different consonants, and every substitute learners reach for is already a real word.
My brother thinks these three things are worth nothing.
One letter pair spells two different consonants here, and English gives no spelling clue about which one you are looking at. Both are made in the same place: the tip of the tongue rests lightly against the edge of the upper front teeth and air is pushed through the narrow gap. The airflow is never shut off and never released as a burst, so it simply keeps running. The only difference between the two is voice. In one, the vocal cords stay still and you hear plain escaping air. In the other they vibrate, and a finger on your throat will feel it. The model sentence above contains seven of them: two voiced, five voiceless.
The sound is rarely dropped. Learners replace it with the nearest neighbour their own language offers, and which one they reach for depends on that language. Many French, German and Slavic speakers pull the tongue back behind the teeth and produce a hiss or a buzz, which turns the last noun of the third example sentence into an unrelated word. Russian, Thai and many other Asian speakers close the gap and make a hard stop. Some urban British and Caribbean speakers move the contact forward to the lower lip and upper teeth. Each substitute is an ordinary English consonant, so the result is not an accent but a different word. Fix the mouth first: tongue tip between the teeth, air still moving, and only then decide whether the throat buzzes.