Present vs. Past subjunctive

Question

Elles ne sont pas allées à la soirée ________ là.They didn't go to the party for fear that you would be there.(HINT: Use 'tu'.)
The correct answer is "de crainte que tu ne sois."Why do we use the present subjunctive instead of theimparfait subjunctive or past subjunctive?
"They didn't go to the party for fear that you would be there."In English, I typically hear "out of fear" vs. "for fear."
Why do we pair le passé composé with the present subjunctive?The past action or inaction was in the past and the fear (of you) wasin the past. I submitted "fusses," but that was incorrect. I assumethat "aies été" was incorrect as well.
Is there a time period where you typically use imparfait (more thana day? or a week?) vs. passé composé with être?

Accepted Answer

Bonjour Deniz,
This is an interesting question!
Here, the main clause is in Le Passé Composé, so Le Subjonctif Présent is used as the action after "de peur que" is still ongoing at the same time as the main clause. 
Le Subjonctif Passé would be used if the action after "de peur que" was over at the same time as the main clause (conjugated in Le Passé Composé).
Take a look here: Sans que (+ ne explétif) + the subjunctive mood (Le Subjonctif) 
Le Subjonctif imparfait is a literary form and is very rarely used these days.
I hope this is helpful.
Bonne journée !