We had a new professor today, who gave us a lecture. We have all seen him around, but we never knew he was a perfect teacher. When we were ten minutes into his lecture , we were laughing as he made lots of jokes from his own experiences. It sounds more natural to not repeat lecture. Experience is correct if he is making jokes about one experience but if he is joking about multiple different experiences it should be plural.

We had a new professor today, who gave us a lecture. We HAVE ALL seen him around. But we never knew he was a perfect teacher. When we were ten minutes into his lecture, we were laughing, as he made lots of jokes from his own experience during the lecture.