Conditions for the validity of Nikaah

Making the condition that the husband cannot take a second wife

Q: I am planning to get married and proposed for a certain girl. The girl whom I want to marry said that if I accept the condition of not taking a second wife in the future, then she will marry me. Personally, I also do not want to take a second wife in the future. I want to spend my life with my first wife till I die. The question is: can we make the condition of not taking a second wife in the future at the time of the nikaah?

Invalid nikaah

Q:

1. A man says to a woman,’Did You marry Rahim?’ The woman replies,’Yes’ in front of many people. Does it mean they are married?

2. A man says to a woman, ’Are you marrying Rahim?’ The woman replies,’Yes’ in front of many people. Are Rahim and the woman married?

Getting married under the age of 18

Q: In Islam, is it halaal to get married under 18 but by Law its illegal to get married under 18?

My question is: Can I do nikah but not get married by Law (I'm under 18 and the girl is under 18 too and we are in Europe... In my country like most of the countries its illegal to get married under 18)

Ma Sha Allah im 17 and she is 16 which means we have to wait 2 years... We both are muslims Ma Sha Allah, and she has a "wali" and everything else. Alhamdulilah its okay but that is the one and only problem.