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Naturally she replied back : Tai sao? A vua moi gap e luc toi ma.
I know she's asking why I'm crazy for her as we only just met.
But can any bro explain to me what is the meaning of luc toi ma?

Luc toi literally means 'at the moment at night'... which can be simplified to be 'that night'.

Later in the afternoon, she smsed to me : E sap ve VN ko con o Sin nua.
A biet e nhieu hon de lam gi? Can anyone help me with this sentence? I can only guess that she's going back to VN soon.

I going back to VN soon. Will not be in sgp anymore. You know more about me for what?

Then in the evening, we made appointment to have dinner near her workplace. But suddenly she smsed to me : E met nen ko di an duoc.
This I really need help as I only know that met=tired and di an=go eat.

I'm tired therefore I can't go eat

I also noticed that most Vn girls like to replace co with ko. Why they like that?

co and ko are 2 different words