Contrary to the cloudy weather forecastforecast that said it would be cloudy, it rained all ththe entire time.

Maybe you can see the reason in this video:https://youtu.be/f0ixdLi-lBs ¶
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One of my favorite Tang Dynasty poets, Wang Wei, once said, "Go to the water ending, sit and watch the clouds rise"--That's also my joy of hiking.

I won't hike today because of theit's raining in eastern China.

This feels a bit more natural.

By now, II've already finished my English learning routine, tookaken care of my little windowsill garden, and cooked a big meal for lunch.

Changing tense like this is a more natural way to include the meaning of "by now" in your sentence.

Oh, I also edited and uploaded a video of the mountain view from my hike last Saturday.

As written is fine. You could also continue this sentence in the present perfect tense, like the last one.

Contrary to the cloudy weather forecast, it rained all ththe whole time.

A bit more natural

ThisIt was a disaster for the hiking process, because it made the trail muddy and slippery.

This is more natural.

Maybe you can see the reason (why) in this video: https://youtu.be/f0ixdLi-lBs ¶
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One of my favorite Tang poets, Wang Wei, once said, "Go to the water ending, sit and watch the clouds rise"--That's also myhow I find joy ofin hiking.

A bit more natural with "why" in the sentence, I think.

By now, II have already finished my English learning routine, tookaken care of my little windowsill garden, and cooked a big meal forsumptuous lunch.

1. The present perfect aspect better conveys the meaning. You’re trying to focus on your present state of having time to spare, rather than the three acts themselves. Wikipedia has an overview of with examples of English sentences: https://en.wikipedia.org/wiki/Perfect_(grammar).
2. Serial comma in this case is not necessary for clarity. But either way is perfectly acceptable to most English users.

Oh, I have also edited and uploaded a video of the mountain view from my hike last Saturday.

Same as preceding comments regarding the perfective aspect.

Contrary to the cloudy weather forecast, it rained all the timeDefying the forecast of overcast skies, it rained continuously.

This was a disaster for the hiking process because it made the trail muddy and slippery.

When we finally got to the top of the mountainreached the peak, I felt it was definitely worth it.

Maybe you can see the reason in this video: https://youtu.be/f0ixdLi-lBs ¶
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One of my favorite Tang poets, Wang Wei, once said, "Go to the water endingProceed to where the water stream ends, sit and watch the clouds rise"-- — That's also my joy of hiking.

Or perhaps “where the water starves itself” would be a more literal translation. But that is not really interpreted in English the same way as “ends”.