Subject/Verb agreement; Pronoun agreement

Fill in the blanks with the appropriate verb or pronoun.

One of the reasons why so many marriages (fail/fails) (are/is) that sometimes people (doesn't/don't) know who (his/their) partner really is. I read that one in five marriages (doesn't/don't) last because a partner is not who (you/one) (thinks/think) (he/they) is. How can this problem be avoided?