the + [short adjective]-est
the most + [long adjective]
the best / the worst / the furthest
A superlative picks one person or thing out of a group: it is bigger, better or more interesting than all the others. The comes before it.
The spelling changes the same way as with -er: old – the oldest, nice – the nicest, big – the biggest, easy – the easiest.
The least is the opposite: the least expensive.
good – the best, bad – the worst, far – the furthest.
With my, your, her and the other possessive words, there's no the.
After a superlative, use in for places and groups (in the world, in my family) and of for times and for a number of things (of the year, of the three).