(from Lernu! (lernu.net))
bonveno (bon·ven·o ← ven·i) welcome
bona (bon·a) good, okay, nice bone (bon·e ← bon·a)
well, OK, okay bono (bon·o ← bon·a)
veni (ven·i) to come veno (ven·o ← ven·i)
(-o marks noun)
-n (accusative case marker)
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Your question is 7 months old by now, but I'm adding this here in case anyone else has the same question:
Esperanto c - English equivalent ts (as in pe_ts_)
There's also a quite good explanation over in English Wikipedia, with descriptions for most of the not-pronounced-similar-as-in-English letters.
I believe it's because it's originally part of a longer phrase.
Why is it "dankon" and not "danko"?
When you use an expression like “Thanks” in English you are actually shortening a full statement. As you will soon learn in Esperanto, -n indicates the object of the sentence as you can see in the examples below:
Dankon = Mi donas al vi dankon.
Thanks = I give you thanks.
("thanks" is the object of the sentence).
Saluton! = Mi donas al vi saluton.
Hi!/Hello!/Greetings! = I give you greetings.
("greetings" is the object)
Feliĉan novan jaron! = Mi deziras al vi feliĉan novan jaron!
Happy new year! = I wish you a happy new year!
("happy new year" is the object)
Someone "ne-komencanto" may be able to fill us in on what the full expression may be ... (my guess: Mi deziras al vi bonvenon! I wish you welcome!)