How to become a strong communicator? How to the little English you know in an effective way?

I think, I reckon, I believe…
今天我们学习的重点就是要看看如何用最简单的单词来表达出自己的看法。

"I think…" is a very neutral way of giving your opinion. "I believe…" is slightly more formal. A very informal way is to say "I reckon…".
I reckon it"s gonna rain later.
"I believe it's going to rain" would sound a little bit formal.
今天的节目中重点介绍了"reckon"这个词,它的意思是"想,认为",比较的非正式.

If you a very strong conviction about sth, you might say sth like "I strongly believe that…", or "I am absolutely certain that…".
If your don't have a very strong conviction about sth, you might say"I'm not so sure,but…"

Ways of responding:
Exactly.(if you absolutely agree with the speaker)

Well, I agree up to a point, but…(if you don't really agree with the speaker)

If you disagree, you can say "I disagree", but it's not very natural. It is better to say "oh, come on…".