Tets and Tavs
Why is there both a tet and a tav in the Hebrew alphabet? They both take a 't' sound. Do they have different purposes depending on the word or context?
You already have one fantastic answer to this question, but I just wanted to remind you that it's common for many languages to evolved to the point where two (or more) letters have the same sound. (For example: c/k - c/s - f/ph. And that's without considering all different ways we spell the same vowel sounds in English) (:
In older times each of these letters had a distinct sound. Today these letters have the same sound, while only a part of the population still pronounces some of them the way they were.
Since there is a fine amount of similar-sounding letters in Hebrew, lets begin by showing them to you.
ת | ט Tav and Tet(h)
א | ע Aleph and Ayin A-yin (Throat letter)
כּ | ק Kaf and Qof
כ | ח Chaf and Chet
ס | שׂ Samekh (Samech) and Sin (not to be confused with Shin שׁ )
ב | ו Vet and Vav (in some cases - not to be confused with Bet בּ ).
Tet used to have a "Th" sound similar to Arabic. Ayin is also similar to the Arabic equivalent.
In most cases you'll just have to memorize when to use which.
Listen how in Arabic you can hear Teth and Ayin, whereas in Hebrew they're gone. Pay attention to the word ארבע, Arba, 4. and Talathe (you can hear the TH).
*They have a few spelling mistakes in the video but it is still useful to us. - I do not recommend to learn through it. :)
I also want to add (to the two other answers) that the pronunciation used by duolingo for Hebrew is the Ashkenazi pronunciation, which is the pronunciation that came from Jewish Europe and the one used by almost all Hebrew speakers today. However, there are other pronunciations of Hebrew that are very similar to the European one, but not quite the same. For example, when listening to Israelis originally from Arab countries, their ע (Ayin) sounds very different from their א (Alef).