the use of the japanese "ga" and "wa"

Last month when Professor Shirai came, he talked about why and how a person is able to learn a second language.

He said that...
-youth
-motivation
-output and input

Which did made me think a bit about how I was able to learn English.
But lets talk about that later.

What I thought that really caught my attention was that "が" and "は" are very hard for people who don't know Japanese.
Not the pronunciation but to when to use the words.

When I thought about it "が" was more in the use of representing something that you are strongly sure of.
On the other hand "は" was the opposite way.

But when I asked the professor he did said I was kind of right but "が" and "は" are more like a past and present-tense in Japanese.

Overall it made me think twice about how language is actually hard to learn and fully comprehend.