你这个问题可能因为时间久了，被新的问题所覆盖，所以今天才看到。

My brother-in-law tried his best to assemble the seat as he saw from the diagrams in the instructions and from his best judgment thought it to be safe. However, the belt buckle mechanism was very easy for my niece to open by herself, and did so on a car journey.

【翻译】按照操作说明图示所看到的，我姐夫尽了最大努力去安装车座，并且根据自己的最佳判断认为这样是安全的。不过，我侄女会很容易地自己打开安全带的扣环系统，并且在汽车旅行中那样做过。

【分析】我认为，though 系thought 之误。在这里，thought it to be safe 与tried his best 是并列谓语，由and 连接：tried his best...and thought...，为了理解方便，可以说thought it to be safe from his best judgment。