Idioms and doubts about them

Hello Guys!

Seguinte, estou precisando de uma mãozinha de vocês. Eu entendo em quais situações posso usar as expressões abaixo, porém não estou conseguindo encontrar uma tradução que fique bacana no Português.

Não liguem para o contexto, o que me preocupa é so a expressão, ok?

1 - They're not sure that want to get married. I think they are getting cold feet.
2 - When you think really hard, you should always be able to know what you feel deep down.
3 - Unfortunately, he is having second thoughts about the car he just bought. It seems that it doesn't get very good mileage.
4 - I am keeping my fingers crossed about getting the scholarship to attend university.
5 - I will risk anything for Tom. In fact, I have gone out on a limb for him a number of times. (poderia ser, talvez, "colocar a mão no fogo" por fulano?)
6 - Mark and Jessica don't agree about anything. They are at odds with each other.
7 - I'm afraid I failed the test. It was so difficult that it was over my head.
8 - It's time to bite the bullet and start studying for the exam.
9 - Jack was climbing the wall waiting for his acceptance letter from the University of Washington.
10 - Please hold off on Tom until we interview the other candidates.

Se não for possível me ajudar em todas, com as que puderem eu já estarei bem contente.
Agradeço desde já pessoal pois, assim como eu, acredito que alguns outros poderão aprender bastante com esse tópico!

Valeu!