Just keep this in mind, I'll try to make it simple:
The feminine article is LA
The masculine article is LO (but only for words begining with vowels, z, s+consonant, and some other forms).
According to the plural rules in Italian, feminine words ending in A, it changes to E, and masculine words ending in O, it changes to I. Thus, in the plural form, LA turns into LE, and LO turns into LI (but, due to pronunciation, actually it turns into GLI).
For most masculine words begining with a consonant different from z, s+consonant, and some others (like PS, or gn), the definite article is IL, and its plural form is I.
Note: contraction is mandatory when the article ends in a vowel, and the noun begins with a vowel. Thus, LA and LO always turn into L' before words begining with vowels.
Hope you understand what I mean.