Hot dDays

They make me feel lazy, and my brain gets all fuzzy.

Also, the water gets lukewarm very quickly and it loses its "refreshing powers" completely.

But, since I like to walk around a little every two hours to stretch and rest my eyes from the computer screen, I have to face the Sun.

Your sentence is correct; when referring to "the Sun," as in the one in our solar system, it is a proper noun.

There are many plants in the garden, and I think that this might also lessen the heat.

Since the beginning of the week, I'm have been watching thisan online course about Java that my company asked me to look into so I could fill in for another worker who is about to geto on vacations.

But today, I can barely concentrate ion what the teacher saysis saying.

TIn the last class, she was correcting some exercises and it was twofor two whole hours long!

In this sentence "she was correcting some exercises" happened "in the last class" so the preposition needed to be changed.

Thankfully, she is explaining about the basics of the language in this first part of the course, and, since I already have some programming background, it' has been easy to follow.

Usually I write out "it's" since it could be "it is" or "it has" and it can be annoying for the reader if they assume the wrong one.

I hope that when I need to tackle more challenging exercises and concepts, the rainy season has already started.

Also, the water gets lukewarm very quickly and it completely loses its "refreshing powers" completely.

I know natives that would say it the way you had it, but this gives the sentence more impact. Maybe there's a grammatical rule, but honestly it's not a big deal.

But, since I like to walk around a little every two hours to stretch and rest my eyes from the computer screen, I have to face the Ssun.

There are many plants in the garden and I think that thisat [they] might also lessen the heat further.

This section is beginning to sound academic. There are so many little things and this is really splitting hairs. I could honestly be wrong - native. ^the second "that" implies another noun here... "I think that [that fact] might lessen the heat. "that fact or idea" refers to "the fact that the plants are in the garden" Or you could just use "they" to refer to the plants. Very complicated thoughts for one word. It literally sounds just fine as I read it initially lol.

SinceFrom the beginning of the week I'mve been watching this online course about Java that my company asked me to look into so I could fill in for another worker who is about to gettake vacations. time (american english),

Bbut today I can barely concentrate in what the teacher says.