The Perfect Tenses

Present Perfect	(a)    Tom has already eaten.	Tom finished eating sometime before now. The exact time is not important.
Past Perfect	(b)   Tom had already eaten when his fiend arrived.	First Tom finished eating. Later his friend arrived. Tom’s eating was completely finished before another time in the past.
Future Perfect	(c)    Tom will already have eaten when his friend arrives.	First Tom will finish eating. Later his friend will arrive. Tom’s eating will be completely finished before another time in the future.