"我的梦想是成为一个科学家" 可以用英语表达为 "My dream is to become a scientist."
"Dream" 是梦想的意思，是一个名词。"Become" 是变成的意思，是一个动词。"A scientist" 是一个科学家的意思，是一个名词短语。
一些例句：
As a child, my dream was to become a scientist and discover new things. (作为一个孩子，我的梦想是成为一个科学家并发现新事物。)
I have always been fascinated by science and my dream is to become a scientist who can make a difference in the world. (我一直对科学感到着迷，我的梦想是成为一个能在世界上产生影响的科学家。)
I want to become a scientist, because I want to contribute to the betterment of humanity. (我想成为一个科学家，因为我想为人类的进步做出贡献。)