1.By illustrating and painting was how Norman was able to make a living
1) 错误：主语只能用名词性的词语充当,如名词、动名不定式和从句.介词短语不能用作句子的主语.
2）修改意见：
* Illustrating and painting was how Norman was able to make a living.插图和绘画是诺曼底人能够借以谋生的方法.（动名词作主语）
* The means of illustrating and painting was how Norman was able to make a living.插图和绘画的手段是诺曼底人能够借以谋生的方法.（名词短语 the means of illustrating and painting 作主语）
* To illustrate and paint was how Norman was able to make a living.加上插图和作画是诺曼底人能够借以谋生的方法.（不定式作主语）
2.During my most recent trip,I came across a wonderful aitique stor wandering in the old quarter of city.
1）语法上没有错误
主要成分：I came across a wonderful aitique stor,
附加成分：
时间状语 -- During my most recent trip
伴随状语 -- wandering in the old quarter of city
2） 原句的意思是：就在我最近的旅行期间,我一边在这个城市的老街区漫步一边偶尔遇到一个惊人的 aitique stor （后两个词可能拼写有误）.
1）come across 是瞬间行为,不可能伴随一个延续性的动作 wander,因此表达逻辑上有偏差.
2）修改意见：
* I was wandering in the old quarter of the city when I came across a wonderful aitique stor .我在正在老街区漫步,这是偶遇一个惊人的 aitique stor.（句型：进行时态主句 + when 引导的用瞬间动词作谓语动词的从句.意思是：正如何,这时候突然如何）