Should has 2 uses in English
1. Asking and giving advice
We use should to ask and give advice:
You should wear sunscreen if you are going to the beach / You should eat more fruit and vegetables if you want to be healthier
Should I bring some drink to the party?
To say that we do not advise something we use shouldn’t:
You should’t spend so much time playing computer games
To give advice about the past we use should have:
I should have listened more when I was at school
In these situations, the action has already been done, so it is too late to change it.
2. Talking about things which we expect to happen
Should is also used to talk about possibility, when we expect that something will happen:
It should be sunny later, they said so in the weather forecast / John should be here by now he said he would be on time
If we say that we do not expect something to happen we use shouldn’t:
It shouldn’t be too cold when we get to the mountains because it is summer
To talk about things which we expect have happened in the past, we use should have:
Our friends should have arrived in Toronto by now, their plane was due to land at 8pm.