-Have you ever thought what will happen in the future? Yes, of course.
I think in the future, our technology will grow at a rapid pace.
I think that one day people will learn to fly. -To fly? Yes.
I believe that people will be able to fly.
First, people make a machine that will be able to move freely through the air. - не согласуются времена - Во-первых, люди изготовили? - тогда made или сделают? тогда will make - подумай еще раз.
And after awhile (пишется слитно, если ты имеешь ввиду: через некоторое время) they can fly themselves. (тогда нельзя can - will be able?)
Scientists have made a device that can lift people into the air. -It's amazing!
But I think that people do not need such invention.
It would be better if the scientists have invented a pen that would do for you your homework.
-I am afraid I do not agree with you. This is a bad idea.
Pupils would be lazy and stupid.
-Do you think scientists will invent a perpetual motion machine? -I do not think it will happen.
'But it would be easier for people to live. -It's a good idea, but it is unrealistic.
-I think scientists can do it. In a few (через) centuries, scientists invent (значит, нужно Future?) a perpetual motion machine.
And humanity will solve many global problems. -
Challenge - is air pollution. Need to invent a device that would catch criminals.
Then everyone would be happy. Peace and good. Yes, it is a good idea.
Science develops (наука развивается - может, здесь Passive? - не сама же она развивается, хотя...), and I think that soon we will see a big change (может быть, big changes - или ты ждешь только ОДНО изменение?) in our lives.