Cases of word usage

1) an examination in order to learn whether something is correct (check)

2) the main meal of the day, whether eaten at midnight or in the evening (dinner)

3) a person who helps the umpire or referee by saying whether the ball crosses a line (linesman)

4) a state of mind or spirit; one of the groups of forms that a verb may take to show whether things are regarded as certain, possible, doubtful etc (mood)

5) a person who is (more) anxious to gain an advantage for herself or himself (than to consider whether it is fair) (opportunist)

6) a feeling of doubt (especially about whether one is doing or has done right); a temporary feeling of sickness in the stomach (qualm)