he is (/hiyIz/)
why he (/waIji/)
/j/, the sound at the begining of "yet", is made by putting your tongue very close to the top of your mouth, as if you are getting ready to say the vowel /i/. That is why this consonant sound is often pron ounced between certain vowel sounds.
Say the phrase "why he" quickly. Native speakers of American English would often pronounce this phrase as "whyje" (/waIji/). Even though it is not spelled that way, we pronounce a /j/ between the /a I/ and /i/ vowels.
/w/ is an interesting sound, because you need to do two different things with your mouth. First of all, you need to make lips round, as if you are getting ready to kiss someone. Next, you need to put the back of your tongue close to the roof of your mouth, almost as if you are going to say g, but DON'T touch the top of your mouth. You must do both of these things at the same time. Use your voice. /w/ is a voiced sound.
Be careful that you do not confuse /w/ with /r/. When you say /w/, your tongue is further back in your mouth and the back of it is close to the top of your mouth. When you say /r/, your tongue is a little bit more towards the front of your mouth, and the tip of your tongue is pointed towards the top of your mouth.
Want to practice more? Contrast the sounds "w" and "y" with this handout.