Under immense pressure at work, she finds she simply could not think straight.

not 是副词，表示否定。 think 是动词，意思是"思考"或"考虑"。 straight 在这里是副词，意思是"清晰地"或"有条理地"，用来修饰 think 的方式。 not think straight 的意思是"无法清晰地思考"或"思绪混乱"，形容在压力、疲劳等情况下难以进行正常的思考。