She was in a mood yesterday and was very unfriendly.

was 是 be 动词的过去式，表示"是"或"处于某种状态"。 in 是介词，表示"处于某种状态或情况中"。 a 是不定冠词，表示"一个"或"某种"。 mood 是名词，意思是"心情、情绪"。 was in a mood 是一个固定搭配，表示"处于某种情绪状态中"，通常指不好的情绪。