Final answer:
The word 'either' correctly completes the sentence about furniture arrival because it refers to one of two possibilities. Examples demonstrate how 'either' is used in decision-making contexts.
Step-by-step explanation:
The correct word to complete the sentence 'The furniture you ordered yesterday is expected to arrive __ or tomorrow' is either. This is because 'either' is used when referring to one of the two choices or possibilities. The sentence suggests that there are two possible days when the furniture might arrive: today or tomorrow. Therefore, the word 'either' fits properly into the blank.
The examples given: 'Either you can go to the movies tonight, or you can go to the party tomorrow.' and 'You cannot go to the movies tonight.' help to understand how 'either' is used in the context of making a choice between two alternatives.
In terms of the information provided about Richard's Furniture Company delivering furniture between 10 a.m. and 2 p.m., this context is not directly related to the grammar question presented but is a different matter related to scheduling and understanding delivery windows.