In Stop or Stopped?

It is mostly used to denote when something is hypothetical or counterfactual. I recommend using stop if you wish to urge people to actually stop something, but stopped if the situation is 'kinda maybe something they could sorta do'.

Is it stop at or stop in?

I want to stop in Barcelona - would be used when you're driving in Spain and go through Barcelona - you want to stop while you're driving through. I want to stop at Barcelona - would be used when you're flying from Madrid to Rome and you want to stop at Barcelona (actually in this case you could probably use in or at).

Is stopped or has stopped?

"Has" + past participle is how we form the present perfect, of the active verb. So "The match has stopped" is very like "The match stopped" (intransitive), but with a perfect instead of a simple past. Perfect is used when the speaker wants to express that an event in the past has some present relevance.