lo = il = the (singular, masculine)
il = before consonant
(l' = before vowel)
lo = before z, sc, sp, sch, ps, pn, x, gn : lo zucchero, lo psicologo, lo specialiste....
Here as "mio" is before "zucchero", the possessive adjective stands before a consonant "m" so it gives : lo zucchero ► il mio zucchero.
Because this is how the verb 'prendere' is conjugated. It is really useful to remember the infinitive form of the verbs you learn. It tells you how the verb is conjugated and the infinitive itself is used quite a bit in Italian. There is a button 'conjugate' underneath the translation in Duolingo. According to the ending of the infinitive (-are, -ere, -ire) the verb is conjugated differently. Look up mangiARE, prendERE, dormIRE (to sleep), capIRE and spot the differences. (it is not very easy to write them all here)