Then and Than :
In some parts of the United States, they are told, then and than not only look alike, they sound alike. Like a teacher with twins in her classroom, you need to be able to distinguish between these two words. Otherwise, they'll become mischievous. They are often used and they should be used for the right purposes.
Than is used to make comparisons. In the sentence :
We have employed the wrong word because a comparison is being made between Piggy's two choices.
We need than instead. In the sentence,
the adverbial construction "other than" helps us make an implied comparison; this usage is perfectly acceptable in the United States but careful writers in the UK try to avoid it (Burchfield).
Generally, the only question about than arises when we have to decide whether the word is being used as a conjunction or as a preposition. If it's a preposition (and Merriam-Webster's dictionary provides for this usage), then the word that follows it should be in the object form.
Most careful writers, however, will insist that than be used as a conjunction; it's as if part of the clause introduced by than has been left out:
In formal, academic text, you should probably use than as a conjunction and follow it with the subject form of a pronoun (where a pronoun is appropriate).
Then is a conjunction, but it is not one of the little conjunctions listed at the top of this page. We can use the FANBOYS conjunctions to connect two independent clauses; usually, they will be accompanied (preceded) by a comma. Too many students think that then works the same way: "Caesar invaded Gaul, then he turned his attention to England." You can tell the difference between then and a coordinating conjunction by trying to move the word around in the sentence. We can write "he then turned his attention to England"; "he turned his attention, then, to England"; he turned his attention to England then." The word can move around within the clause. Try that with a conjunction, and you will quickly see that the conjunction cannot move around. "Caesar invaded Gaul, and then he turned his attention to England." The word and is stuck exactly there and cannot move like then, which is more like an adverbial conjunction (or conjunctive adverb — see below) than a coordinating conjunction. Our original sentence in this paragraph — "Caesar invaded Gaul, then he turned his attention to England" — is a comma splice, a faulty sentence construction in which a comma tries to hold together two independent clauses all by itself: the comma needs a coordinating conjunction to help out, and the word then simply doesn't work that way.