My Favourite Subject I love

I think for me, my favourite subject iswould be biology.

I have loved this subject since I was very young.

When I was in primary school, we had this subjectwere taught this in my school.

At that time, it was quite easy because these books just showed us colorful pictures of animals.

"these books" suggest the books are in front of you currently

For example, the books showed some interesting animals we haved never seen in our daily life, which can easily catchught my eyes.

The sentence should be in the past since you're talking about when you used to be in school

On top of that it was as easy as a-b-c, so that I could keepkept up with teachers and make thegot good grade s, that can makde me feel a sense of fulfillment at that time.

Again, it should be in past tense. I wouldn't say "make the grade" since that doesn't make much sense in English, instead "achieved good grades", "got good grades" sound better.

Then when we went to secondary school, and it became a little bit more difficult because it introduced the difference between men and women, the organs, how women get pregnant and how they deliver a baby.

When I attended high school, it became much more difficult and complicated because it liked mathematics and the teachers covered a lot of ground at once.

Every time when it came to the pop quiz, my mind went blank and I would bombed my tests at that time.

the "at that time" is unnecessary

So I stayedgot behind in this subject.

You weren't behind beforehand but became behind hence why I used "got" instead of "stayed" since "stayed" implies that you were behind before.