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1. Liam is swimming in the pool. He started swimming half an hour ago.

Liam has been swimming in the pool for half an hour.

2. Dave has had this computer for three weeks.

   Dave bought this computer three weeks ago.

3. They went to the supermarket at 6:00 pm. Now, it's 7:00 pm.

    They have been in the the supermarket for an hour.

4. It's 10:00 am and I'm about to start writing my fourth letter.

  I have written three letters so far.

5. I started training five months ago. Now, it's August.

    I have been training since March.