For those interested in Semetic roots: So עָצָב means a "NERVE" in Heb, in cousin language Arabic it's exactly the same word عصب and same pronunciation. But the Hebrew adj עָצוּב "sad" is 'haziin' or 'za'laan' in Arabic. However the root עצב = عصب pops up again in the verb: bit'aSab 'alay (بتعصب علي) in Arabic is to get upset/angry with s/o or to be intolerant of them.
Indeed, the sound is often approximated by 'ts' or 'tz'. It is also the same as the sound of letter 'Z' in German, as in 'Mozart'.
Why doesn't one pronounce "sad" in hebrew always "aht soov" instead of just "soov" in this sentence? "Zeer soov" This is sad. Couldn't one say: "Zeer aht soov" for example. Why is the "ע" in this case silent?
Pay attention to the exact letters and the sounds represented by them:
'זה' = ZEH / ZE (no 'R' sound is present')
'עצוב' = ATZUV / ATZOOV (a single word; 'ע' is corresponds here to the 'A' sound, 'צ' to the 'TZ' sound, 'ו' to the 'U' / 'OO' sound, 'ב' to 'V')
In normal speech, the vowel sound at the end of a word may often be connected to the word that follows. This may explain to some extent why it seems tricky.
I hate it when they introduce nrw words without preparation. How is one to guess?