Question

when she came i (write) a letter​

Answer 1

WHEN SHE CAME, I WAS WRITING A LETTER..

hope this helps you.....

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Answer 2

When she came i was writing a letter​ .

Explanation:

Incomplete sentence:

When she came i (write) a letter​ .

Complete sentence:

When she came i was writing a letter​ .

This is a two clause sentence. As the first clause is in simple past tense the according to the rules the second clause must be in past continuous tense.

It means the work was in progress when something else happened in past.