2,628. Answering "Amen" to an Oath

Hilchos Shevuos 1:13

An example of acting unintentionally vis-à-vis an oath of testimony is if one forgot that making a false oath would obligate him in an offering, but he knew that the oath was prohibited and that he would be swearing falsely; intentional violation is when one knows that he must bring an offering for swearing falsely. If one didn’t know that a false oath is prohibited, or if he forgot his testimony and took an oath, only to find later that he knew testimony and swore falsely, he is considered to have acted based on circumstances beyond his control and he is exempt from bringing an offering.

Hilchos Shevuos 2:1

A person is liable for taking one of the four types of false oaths regardless of whether he did so on his own or was placed under oath by someone else and he only answered “amen.” This is the case even if the oath to which answered amen was imposed by a non-Jew or a minor. This is because one who answers with amen or with an equivalent response like “yes,” “I am bound by this oath,” “I accept this oath upon myself,” etc. – in any language – is considered to have taken an oath in all ways, including liability to the penalty of lashes or to bringing an offering.