grammer questions

[yamini]

This book is not found - is correct? or
This book was not found - is correct?

same way

It is completed - is correct? or
it was completed - is correct?

[StaceyKoprince]

If you have a particular test question in mind, you might want to post it. The only distinction in your pairs of sentences is that the first one (of each pair) is in the present and the second one is in the past. Whether you use variations on past or present tense just depends upon the rest of the sentence - the context of the sentence will tell you whether you need to be in past or present.

You probably wouldn't say something like "This book is not found" however - you'd probably say "The book has not been found" or "The book was not found." First, "this" implies that you are pointing at the book... and if it's lost, you can't point at it. :) Second, you already know that the book hasn't been found, so you wouldn't put it in just straight present tense, which would indicate that you are in the act of not finding it - that doesn't make any sense.