I heard this recently during a radio broadcast.

Problem:
The word “then” is not a preposition.

Explanation:
The radio broadcaster said a sentence such as “The telephone calls to the radio station during this hour were no more frequent then usual.”

The problem with this sentence is that the word “then” — spelled T-H-E-N — is not a preposition.

Instead, this word is any of the following:

• adverb, as in “Prices were higher then.”, where “then” means “at that time” in this sentence;
• adjective, as in “The then president of the club was a nice guy.”, where “then” means “existing” in this sentence;
• noun, as in “We have not seen a show at the Alley Theatre since then.”, where “then” means “that time” in this sentence.

What the phrase “no more frequent then usual” requires is a preposition, given that the speaker is comparing “more frequent” with “usual”.

The required preposition is “than” — spelled T-H-A-N, not T-H-E-N.

I believe that the common English blunder of using “then” where the preposition “than” is required is due in part to mispronunciation of the preposition “than”, but simple ignorance about these two words certainly could play a part, too.

Solution:
“… no more frequent than usual.”