The error in the sentence "I prefer tea than coffee" is the use of the word "than". The verb "prefer" indicates a choice or a liking for one thing over another. In English grammar, when expressing a preference between two nouns or noun phrases, the correct preposition to use after "prefer" is "to", not "than."
The structure for expressing preference with "prefer" is typically: "prefer [something] to [something else]". For example, one would correctly say, "I prefer apples to oranges," or "She prefers reading to watching television." While "than" is commonly used with comparative adjectives (e.g., "taller than," "better than"), it is not appropriate after the verb "prefer." Using "than" in this context is a common grammatical error that can lead to confusion and incorrect sentence structure. Therefore, to make the sentence grammatically correct, "than" must be replaced with "to," resulting in the correct statement: "I prefer tea to coffee." This rule ensures clarity and adherence to standard English usage when expressing preferences.