It was so thoughtful of you to remember my birthday with a card.

thoughtful 是由名词 thought（想法、思考）加上形容词后缀 -ful 构成的形容词，意思是"考虑周到的、体贴的"。后缀 -ful 表示"充满...的"，所以 thoughtful 字面意思是"充满考虑的"，引申为"体贴的"。