The following sentence has a blank that must be filled with an appropriate word. Choose the correct alternative to complete the sentence: I waited _____ the train arrived.

Introduction / Context:
This is a basic sentence completion question that tests correct use of conjunctions expressing time. Such questions are common in competitive exams because they check whether the candidate can choose the right linking word to show the relationship between two actions. Here, the sentence describes one action waiting and another action the train arrival. The word we select must correctly show that the waiting continued up to the moment when the train came.

Given Data / Assumptions:
The sentence reads I waited _____ the train arrived. The answer choices are as, until, since, and after. Only one of these conjunctions correctly expresses the intended meaning in standard English, where waiting continues up to a particular time. We assume that the speaker is talking about a past event in a simple narrative style.

Concept / Approach:
The conjunction until is used to show that an action continues up to a specific point in time and usually stops at that point. In the sentence I waited until the train arrived, the meaning is that the waiting did not stop before the train came; it ended at the moment of arrival. The other conjunctions express different kinds of time relationships: as often means while or because, since shows reason or starting point, and after refers to a time that follows an event. None of these exactly match the idea of continuing up to a moment and then stopping.

Step-by-Step Solution:
Step 1: Understand the meaning of the base sentence: the speaker continued to wait and finally the train came. Step 2: Recall that until is used when an action goes on up to a certain point in time. Step 3: Place each option into the blank and read the sentence aloud: I waited as the train arrived, I waited until the train arrived, I waited since the train arrived, I waited after the train arrived. Step 4: Notice that only I waited until the train arrived sounds natural and conveys the idea of continuous waiting that stops when the train appears. Step 5: Select until as the correct conjunction for this structure.

Verification / Alternative check:
To verify, we can compare related sentences. For example, He worked until midnight shows that work continued up to midnight. Similarly, She stayed in the office until the manager left. In each case, until introduces the time up to which the earlier action continues. If we try to insert as, since, or after in those sentences, the meaning changes or becomes incorrect. This supports our choice of until for the question sentence about waiting and the arrival of the train.

Why Other Options Are Wrong:
As often means while or because, for example, As the train arrived, it started to rain. That pattern does not suit the idea of waiting up to a moment.
Since can introduce a starting point in time or a reason, as in I have been waiting since morning or Since it was late, he went home. It does not fit directly into the given structure.
After refers to a time that follows an event, as in After the train arrived, I went home. The sentence I waited after the train arrived would suggest that waiting began at that point, which is not the intended meaning here.

Common Pitfalls:
Learners sometimes confuse until with after or since because all of them relate to time. It helps to remember that until focuses on the end point of a continuing action, after focuses on what happens later, and since often focuses on the starting point of an ongoing action. Keeping these distinctions clear will make time based sentence completion questions much easier to handle in exams.

Final Answer:
The correct word to complete the sentence I waited _____ the train arrived is until.