Understanding Prepositions of Time for Duration

The question asks us to replace the underlined part "from two hours" in the sentence "I was waiting from two hours." with the most appropriate option. This sentence is about indicating the duration of time that the waiting action has been happening.

In English grammar, specific prepositions are used to indicate time, including duration. Let's look at the options provided and how they are typically used with time expressions.

• since: Used to indicate a point in time from which an action began and continued (e.g., "since morning", "since 2010", "since she left"). It is used with a specific point, not a duration.
• in: Can indicate a period within which something happens (e.g., "in the morning", "in June") or a future time when something will be completed (e.g., "I will be back in an hour"). It is not used to indicate how long an action has been ongoing in the way the original sentence intends.
• for: Used to indicate a duration of time (e.g., "for two hours", "for three days", "for many years"). It answers the question "How long?". This fits the context of the sentence which specifies a duration of "two hours".
• at: Used to indicate a specific point in time (e.g., "at 7 PM", "at noon", "at sunrise"). It is not used for duration.

The original sentence "I was waiting from two hours." uses "from" incorrectly to specify the duration. The preposition "from" is typically used with a starting point in time (e.g., "The store is open from 9 AM to 5 PM") or location. To express the length of the waiting period, the correct preposition is "for".

Therefore, the most appropriate option to substitute "from two hours" is "for". The corrected sentence would be "I was waiting for two hours."

Let's analyse the options based on this understanding:

Based on the analysis, "for" is the only preposition that correctly indicates the duration of "two hours".

Revision Table: Prepositions of Time

Additional Information on Prepositions of Time and Duration

The use of "for" and "since" is particularly common with perfect tenses (present perfect, present perfect continuous, past perfect, past perfect continuous) to indicate duration up to a certain point. However, "for" can also be used with simple tenses or continuous tenses, as in the example "I was waiting for two hours," to state the duration of the action.

• Present Perfect/Perfect Continuous: I have waited for two hours. / I have been waiting for two hours. (Waiting started two hours ago and continues now).
• Past Continuous: I was waiting for two hours (when something else happened). (Waiting happened for a duration of two hours in the past).

It's important to distinguish between duration (how long something happens) and the starting point (when something began). "For" answers "How long?", while "Since" answers "From when?".

In the original sentence, "two hours" specifies the length of the waiting period, making "for" the correct choice.