Here are the corrected tenses for each sentence:

1) We **finished** our dinner half an hour ago.
2) The farmers **are working** in the field now.
3) Mrs. Green always **goes** to work by bus.
4) My parents **have lived** in London. They **have been** there for 20 years.
5) She paid for her ticket and **left**.
6) She **was sitting** under a tree when it **began** to rain.
7) How many times **have you seen** him since he went to Edinburgh?
8) Mr. Davy **has worked** in the same place for thirty years and he is not planning to change.
9) Rivers usually **flow** to the sea.
10) Do you know that man who **smokes** there?
11) My brother **has worked** in this factory since 2005.
12) I **washed** my hair before breakfast this morning.
13) What **was** Tom **doing** when you saw him?
14) How often do you **read** a newspaper?
15) He **has played** football since he **was** ten years old.