‘Amen’ is a Hebrew word that was never really translated into English, it was just anglicized (made to sound English). In Greek, it’s pronounced, ah-main. People never translated it because scholars were afraid to change this supposed ‘magical’ word that ensures that God will listen to our prayers.
Interestingly, there is no record in the Bible of anyone ending a personal prayer with amen. However, we do know that it was likely said at the end of group prayers. At 1 Corinthians 14:16, Paul mentions that hearers of a congregational prayer would say amen at the end to show that they agreed.
However, all other uses of amen in the Bible (especially by Paul) have nothing to do with prayers; they are merely exclamations to affirm that what the person is saying is true (like saying ‘absolutely!’ or ‘right on!’). So the literal meaning and best translation of the word amen would be ‘May it be so’, and this is how it usually appears in our translation.
Then, should Christians end their prayers with the word amen (or with may it be so)?
It really makes no difference. There is no specific instruction about it in the Bible. Further, God is surely smart enough to know when you’ve ended your prayer! However, an audible ‘amen’ (or, ‘may it be so’) at the conclusion of a public prayer is surely useful, as it indicates that the prayer has indeed ended and that we agree with the words that were spoken (if, in fact, we do agree).
In our translation, we’ve left the word amen unchanged in just four places, each for poetic reasons. The only place in the Christian books is at Revelation 3:14, where Jesus is referred to symbolically as ‘the Amen.’