Open the brackets and use the past perfect tense
We (see) him before they (make) friends with him
I already ( get) home before it (begin) rain
There (be) a new painting That (I not see) before
Captain Cook (discover) the island before he (die)

We had seen him before they made friends with him. I had already got home before it began raining.There was a new painting That I had not seen before. Captain Cook had discovered the island before he died.