The error in the sentence "He is senior than me" lies with the word "than". This is a common grammatical mistake involving comparative adjectives of Latin origin. Words like "senior," "junior," "superior," "inferior," "prior," and "preferable" are inherently comparative and do not follow the standard English comparative structure that uses "than." Instead, these adjectives are always followed by the preposition "to".
In English grammar, when comparing two entities using these specific Latin-derived adjectives, "to" is the correct preposition to establish the relationship. For instance, one would say "He is senior to me," "She is junior to him," or "This product is superior to that one." Using "than" in these contexts is incorrect because "than" is typically used with English comparative forms like "taller than," "smarter than," or "more beautiful than." Therefore, to correct the sentence, "than" should be replaced with "to," making the grammatically correct sentence: "He is senior to me." Understanding this rule is crucial for precise and formal English usage.