that depends on what is after during,and there are 3 basic types. 1. it's an experience after during. e.g. i like to play video games during my free time. we use character "当....的时候” after the event. 当我有空（my free time）的时候，我喜欢玩电玩(i like to play video games)。 2. it's an event after during. e.g. there was an arms race between the USA and Russia during the cold war. we use character"期间” after the event. 冷战（the cold war）期间(during),美国和俄罗斯之间有军备竞赛。 3. it's a period of time e.g. we go swimming during summer. we use “的时候” after the time. 夏天(summer)的时候，我们去游泳。 for sentence structure, the time is usually at the beginning of the sentence.

During my free time （当）我有空的时候

Hi There , during your free time u should write as ”有空“ or 的时候 ，时， as the following sentence will be translated as "when i'have time ,i'going to language school to practice Chinese , "我一有空，就去语学练习中文"。 another example - 我觉得很累的时候，更喜欢喝咖啡。 when i'm get tired 'preffer to drink coffee. about the strcucture userlly the time will come after the subject .... good luck