After I thought about it for a long time, I understood I would have to get an education if I wanted to be successful in life.

After I thought about it for a long time, I realized I would have to get an education if I wanted to be successful in life.

I realise maths because it was my major at university

I understand maths because it was my major at university

I understand why Mary is so angry about not getting a promotion because the same thing happened to me.

I realise why Mary is so angry about not getting a promotion because the same thing happened to me.

I realized that I had left my cell phone at home when I put my hand in my pocket.

I first understood how hard it is to speak another language when I took a Spanish class about five years ago.

I first realized how hard it is to speak another language when I took a Spanish class about five years ago.

Which is correct?