Questions & Answers about כשעליתי על הסולם ראיתי חור קטן ליד החלון.
כש־ means when. It is a very common shortened form of כאשר.
In Hebrew, it is often attached directly to the next word, so:
כשעליתי = when I went up / when I climbed up
This is very normal, everyday Hebrew.
עליתי is the 1st person singular past tense of לעלות, which means to go up, to climb up, or to get on depending on context.
The ending ־תי is the usual past-tense ending for I:
- עליתי = I went up / I climbed up
So in this sentence, כשעליתי על הסולם means when I climbed up onto the ladder or when I got on the ladder.