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Hello,
Could you check my corrections, please?

How sad! George (dream) dreamt of going to California before he died, but he didn't make it. He (see, never) never saw had never seen the ocean.

This mountain (be, never) has never been climbed by anyone. Several mountaineers (try) have tried to reach the top, but nobody (succeed, ever) has ever succeeded. The climb is extremely difficult and many people (die) have died trying to reach the summit. Can I use past simple in each sentence here?

15. I (visit, never) have never visited Africa, but I (travel) have traveled to South America several times. The last time I (go) went to South America, I (visit) visited Brazil and Peru. I (spend) have spent two weeks in the Amazon, (hike) have hiked for a week near Machu Picchu, and (fly) flew over the Nazca Lines.

Should 'the last time' always be followed by past simple and in the last hundred years by present perfect? Eg., . In the last hundred years, traveling (become) has become much easier and very comfortable.

Thanks
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This one is wrong: He (see, never) never saw had never seen the ocean.

Can I use past simple in each sentence here?-- No. Mountains will eventually be climbed.

Should 'the last time' always be followed by past simple and in the last hundred years by present perfect?-- No
Eg., . In the last hundred years, traveling (become) has become much easier and very comfortable.-- Yes.
Comments  
Should have been possible to use past perfect for the first verb because both wishes dreaming and seing took place before his dying

How sad! George had dreamt of going to California before he died, but he didn't make it. He (see, never) never saw had never seen the ocean.

THANK YOU FOR YOUR ANSWER