Come to think of it, it really is quite difficult to say.

come 是动词，意思是"来"，在这里构成短语的一部分。 to 是不定式符号，用于动词原形前构成不定式。 think 是动词，意思是"思考、想"。 of 是介词，意思是"关于"。 come to think of it 是一个固定短语，意思是"仔细想想"或"说起来"，用于引出突然想到的事情或观点。