Hello! Thanks for the reply. I checked what you mentioned and yes, you were right.
I tried to encode the sentence at the top setting add_special_tokens=False and True.
Token 4 is <s>, which is both the <cls> token and the <bos> token, while token 6 is </s>, which acts as sep_token and eos_token.
The first case has add_special_tokens=False and its special token mask is full of 0’s, the first case has add_special_tokens=True and as expected the <bos>  and <eos> tokens were added by the algorithm. The special tokens mask only shows the first and last tokens as “1”, while all the other 4’s and 6’s are missing.
<s> ID 10 </s><s> NAME Trevor </s> <s> COUNTRY USA </s><s> CAPITAL Washington DC </s>
[4, 0, 232, 28, 27, 6, 4, 1, 232, 63, 93, 80, 97, 90, 93, 6, 4, 2, 232, 64, 62, 44, 6, 4, 3, 232, 66, 76, 94, 83, 84, 89, 82, 95, 90, 89, 232, 47, 46, 6]

[4, 4, 0, 232, 28, 27, 6, 4, 1, 232, 63, 93, 80, 97, 90, 93, 6, 4, 2, 232, 64, 62, 44, 6, 4, 3, 232, 66, 76, 94, 83, 84, 89, 82, 95, 90, 89, 232, 47, 46, 6, 6]
[1, 0, 0,   0,  0,  0, 0, 0, 0,   0,  0,  0,  0,  0,  0,  0, 0, 0, 0,   0,  0,  0,  0, 0, 0, 0,   0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,   0,  0,  0, 0, 1]