Question

Add a subordinate clause to expand the sentence meaningfully.   

1. I found a pen which was lost yesterday.
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Answer 1

I found a pen which lost by me yesterday

Answer 2

The answer is: It was me that(I) found a pen, which was lost yesterday. The sentence was divided into two clauses:

I found a pen and which was lost yesterday.  

For the expansion of the sentence and the clarification of the meaning, one new clause is added, “it was me”.

Now the sentence is made of three clauses.

One is It was me, second is who found a pen, and the 3rd is which was lost yesterday.