That's not quite right. Can nothing else be done? sounds more natural. or Is there nothing else that could be done?

Hello Friend! How are you? I am very good! I hope you have a great day! See you soon, Diana

This needs more context. Maybe the story says "She told me her mother would not be home. When I went by her house, though, her mother's car was still there." So then you know that "though" means that you did not think the car would be there, though it was.

This is VERY good! You spelled insignifficant wrong - it is insignificant. And you spelled genreral wrong - it is general. Also I would say, "First," instead of "Firstly" but that may be American vs British? I would say "also had slight changes in 2010" in the fourth sentence.

They all mean the exact same thing. They can be used interchangeably. As a native speaker, in casual conversation, most people would say "I'm going to play tennis tomorrow". If someone says "Can you come to the party?" You might answer "Oh I can't, I'll be playing tennis tomorrow.

Investing means to buy something. To put your money into it, like the stock market. You could also say "buy in". A hot stock or investment idea is something like Bitcoin. Its the next "hot" investment idea. But you shouldn't buy a lot of it, its dangerous. Its risky to invest all your money in a new thing. You will waste all your years of careful planning.

I will make sure you understand it very well. or I will make sure you understand me very well. or I will help you understand it very well.

Most English speakers would say #2 sounds better, but they are the same. The difference is basically the placement of "not" because "don't" is a contraction of "do not". So the sentences could be: 1. I think it's not easy for you to conquer Mt. Everest. 2. I do not think it's easy for you to conquer Mt. Everest. You could also replace "not easy" with its opposite word, like difficult, ie "I think it would be difficult for you to conquer Mt. Everest.