A: This is a very common, and complicated, situation which affects almost all people on a daily basis.
Here are a few basic guidelines with which to determine your personal status:
1) Borei Nefashos is only made if one eats a kzayis of food or a revi'is of beverage (except wine or grape juice, which needs a Al HaGefen), within a certain time span (between 3 and six minutes, depending on different Poskim)
2) Borei Nefashos should ideally be said in the place where you ate/drank immediately following the eating/drinking, but may be said in a different location as well, and up to 72 minutes after consuming food [though you have significantly less time after drinking. According to Rav Scheinberg Shlita you ony have 30 minutes after a drink, unless you have become thirsty again, in which case the first drink is no longer satisfying you, and then you cannot make a Borei Nefashos anymore, even if it's less than 30 minutes since you drank].
3) If one is not sure if he/she is required to make a Borei Nefashos, since it is M'Drabanan, it is best to not say it. (this applies to one who forgot if it was already said as well as to one who is uncertain if he ate/drank the minimum needed in the right amount of time to qualify for requiring the Bracha Achrona)
4) In the above case, where you aren't going to say Borei Nefashos, try one of the following things instead: 1) Eat another Kzayis of food/Revi'is of drink and then make Borei Nefashos on everything. 2) Ask someone else who is making a Borei Nefashos,to be Motzi you, and answer Amen 3) Think the words of the text in your mind without saying them, as this is acceptable according to the Chazon Ish based on the Rambam.
I hope these guidelines help you determine the right thing to do in your situation.
For a more lengthy treatment of this topic see "Halachos of Brochos" by Rabbi Yisroel Pinchos Bodner, an excellent Sefer that I think every Jewish home should posess.
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