Verb conjugation of "sniff" in English

I bought the ticket very quickly and ran to the gate. A stewardess was waiting for me and waved. She shouted that I should hurry up. I ran as fast as I could and entered the gate leading to the plane.
You have to be careful when you go on summer vacation. There are many dangers but the most dangerous one is the sun itself, because if you're not careful it may harm you. People underestimate the rays.
Now I am learning by other means, such as reading and writing. I can not begin with conjugation training because I lack the basic knowledge. How are you learning at this site and which is your best lesson?