Here, lemme try to explain:
When you form the genitive (possessive) form of a word in English, you add 's. But if that word already ends in -s, like "girls" or "Marcus" then you'd get "girls's" "Marcus's", but that second s is not said, so you leave it away in spelling and only put the apostrophe to show that it's a genitive form:
girl -> girl's girls -> girls's -> girls'
Not true. In Spanish you say "del" for "de el". You say "al" for "a el". You don't form a contraction with the feminine because the feminine article is "la" and contractions only form when the the first word ends with a vowel and the second word begins with a vowel. I believe that "del" and "al" are the only Spanish contractions but I could be wrong.