In most russian grammar books there is a rule saying that definite article must be used with superlative adjectives. However from time to time I see people using indefinite article. For example, a title of the film "A Most Wanted Man". Can somebody explain this?
Generally, no. As mentioned in the comments, phrases like "a tallest mountain" or "a biggest dog" do not sound sensible.
However, the use of "most" as an adverb does not necessarily imply the superlative; it can be merely intensive. From Merriam-Webster on most:
1: to the greatest or highest degree ... (the most challenging job he ever had)
2: to a very great degree (was most persuasive)
The first definition implies the superlative, but the second definition is merely an intensifier. The phrase "a most wanted man" (by the second definition) is equivalent in meaning to "a very wanted man".
The superlative degree has two meanings: the highest degree of manifestation of the attribute ('the most') and a high degree ('a most').
In the first case, a limiter is needed, e.g. She is the most beautiful girl in the class/in the world/ of the year/I have met (i.e., a circumstance of time, place, designation of a group of people or a subordinate clause (often in the perfect tense); the limiter may be contextual).
In the second case there is no limiter, e.g. She is a most beautiful girl. (to a very high extent, extraordinally). Here the stress is on the article, i.e. it is pronounced as /eɪ/ (strong form).
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