GI1 - Past simple or present perfect

What (have they seen/ did they see) at the art gallery last Monday?

When Helen was small, her mom often (didn't encourage/ hasn't encouraged) her to learn a foreign language.

He (hasn't understood/ didn't understand) the lesson yesterday because it (was/ has been) too difficult.

Mike (has just decided/ just decided) to spend this weekend on relaxing after a long business trip.

The teacher (reminded/ has reminded) Paul about his homework several times.

(Have the police investigated/ Did the police investigate) the robbery yet?

It's a month since Philip last (dropped in/ has dropped in) this bookshop.

(Has Lisa ever been/ Did Lisa ever been) to Phi Quoc island? - No, she hasn't/ didn't

Emma is the most intelligent person they (have ever talked/ ever talked)