Incorrect. 'while' means at the same time as."While Barbara puts in her contact lenses, the telephone rang".
The word 'while' here is talking about present tense and so we use the word 'rang' as the telephone had rung, right?
'have expressed' doesn't fit."By the time negotiations began, many pessimists have expressed doubt about them".
I have changed 'began' into 'had begun', am I right?
It's awkward."The moderator asks for questions as soon as the speaker has finished."
I think the sentence above has the appropriate tenses, am I right?
"Everyone hopes the plan would work."
It should be "hoped" or you should change as "the plan will work" if you do not want to change 'hopes' to 'hoped'. Again, am I right?[/quote]'hoped'. Right!
Work on the others.
All the best.
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