Here are the correct 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 **live** in London. They **have been** there for 20 years.
5) She paid for her ticket and **left**.
6) She **sat** 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 retire yet.
9) Rivers usually **flow** to the sea.
10) Do you know that man who **smokes** there?
11) My brother **has been working** 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 **played** football since he **was** ten years old. (This may need "he has played football since he was ten years old" for clarity, depending on the intended meaning.)

Feel free to ask if you need further assistance!