Un, an easy way to tell is through the ending vowel.
The 'o' in boligrafo is masculine, so the there are two automatic answers brought forward, 'un', and 'el'.
Then, in english, it would be like saying 'the' or 'a' whereas the is either 'el' or 'la', while a a 'un' or 'una' respectively.
The translation is:
Interesting text considering punctuation marks, connectors, Coherence, speech markers and minimum 40
Mountains are the physical boundary between Chile and Argentina
