I could get you a concert ticket. → I could not / couldn't get you a concert ticket.
I was on the Internet when you called me. → I was not / wasn't on the Internet when you called me.
【技巧3】当句中谓语是除情态动词、助动词、系动词was,were以外的动词时，在该动词之前加did not / didn't，动词还原，构成否定句。例如：
The famous singer sang some Chinese songs. → The famous singer did not / didn't sing any Chinese songs.