To be fully aware of or to gain knowledge.
Past Participle form
I always realise at the last minute.
She never realises she burns the dinner.
A: I always realise too late.
B: What do you mean?
A: Today I didn't realise that the dogs water was empty.
B: Oh, so you didn't see it.
A: I saw it but was not aware of it.
I realised she wasn't happy.
We realised quickly that we'd be late for the film.
A: I realised she wasn't happy when she was quiet at work.
B: What did you do?
A: I spoke to her about her work and home life.
I am realising that I have made a terrible mistake.
They were realising that the bus wasn't coming after they'd waited for 40 minutes.
A: Are you realising your mistake?
B: Kind of. Was I meant to add a preposition in here?
A: Yes, very good.
Have you realised what it wrong?
I had realised that I was at the wrong theatre but it was too late.
A: I have realised what was the key to success.
B: What was it?
A: Having an online shop.