**1) i (write) the letter last night.**

The correct form of the verb "write" to complete the sentence is "wrote."

**2) The First World War (Last) for four years and ended in 1918**

The correct form of the verb "last" to complete the sentence is "lasted."

**3) The Rubbish van (come) again in the afternoon tomorrow.**

The correct form of the verb "come" to complete the sentence is "will come."

Let's break down the explanation for each sentence:

**1) i (write) the letter last night.**

- The sentence is written in the past tense, indicating that the action of writing the letter occurred in the past.
- The subject of the sentence is "I," which refers to the person speaking or writing.
- The correct past tense form of the verb "write" is "wrote."
- Therefore, the correct sentence is: "I wrote the letter last night."

**2) The First World War (Last) for four years and ended in 1918**

- The sentence is describing the duration and conclusion of the First World War.
- The verb "last" needs to be in the past tense to match the timeframe of the war.
- The correct past tense form of the verb "last" is "lasted."
- Therefore, the correct sentence is: "The First World War lasted for four years and ended in 1918."

**3) The Rubbish van (come) again in the afternoon tomorrow.**

- The sentence is referring to a future event, indicating that the rubbish van will come again.
- To express a future action, we need to use the auxiliary verb "will" before the base form of the main verb.
- The base form of the verb "come" is "come."
- Therefore, the correct sentence is: "The rubbish van will come again in the afternoon tomorrow."

Overall, by using the correct verb forms, we can ensure that the sentences are grammatically accurate and convey the intended meaning.