`1` were ( in the past 3 days - thì quá khứ đơn , 3 accidents - số nhiều -> dùng were ) 

`2` found ( in the last 50 years - thì quá khứ đơn , S + Ved / V2 + O ) 

`3`  has improved/ left ( HTHT + since + QKD ) 

`4` bought ( last week - thì quá khứ đơn ) / haven't worn ( yet - thì hiện tại hoàn thành ) 

`5` haven't seen / left (HTHT + since + QKD ) 

-> Thì quá khứ đơn : S + Ved / V2 + O 

-> Thì hiện tại hoàn thành : S + has / have + Vpp + O 

# Nami (◡ ω ◡)

1. There was 3 accidents on the street in the past 3 days (In the last dùng QKĐ)

2. Scientists found cures for many illnesses in the last 50 tears (In the last dùng QKĐ)

3. My uncle's health has improved since he left India ( HTHT since QKĐ)

4. I bought a new shirt last week but I haven't wore it yet (last week dùng QKĐ, yet dùng HTHT)

5. I haven't seen Lan since we left school ( HTHT since QKĐ)