What should I do if I don't know a word when doing the translation section of the exam?

The important thing is not to panic! It doesn't matter if you don't know or can't remember a specific word given in the passage. The exercise is designed primarily to test your knowledge and usage of grammar and syntax rather than how well you have memorised lists of words. You can still gain marks if you show you are flexible and can convey the same/a similar meaning. Try and be creative and think of synonyms in English to help you. For example, if the exam asked you to translate 'at the dawn of the 20th century', but you couldn't remember the word for 'dawn', you might substitute this with 'at the beginning of the 20th century' ('au début du vingtième siècle'). The meaning is essentially the same and should still get you marks. The golden rule is: always write something - never leave a blank space!