Learning a new language takes time, but there are ways to make it easier. First, practice a little every day instead of studying for many hours once a week. Regular practice helps the brain to remember new words. Second, listen to the language as much as possible. Songs, news and films will help you to get used to the sounds. Third, do not be afraid of mistakes. Everyone makes them, and they are a normal part of learning. Speaking with others, even badly, is better than staying silent. Many learners also keep a notebook for new words and review it often. Finally, set small goals, such as learning five words a day. Small successes keep you motivated, and after a few months you will be surprised by how much you have learned.

Question: In the sentence 'review it often', what does 'it' refer to?