Student or Learner
He was executed as a criminal after (a) trial in court.
Is 'a' required before the word 'trial' ?
I am not a teacher.
It is not essential. You'll find examples without the article at http://fraze.it/n_search.jsp?q=%22after+trial%22&l=0
I find some of the words in the original redundant. Generally, we don't execute people unless they have committed a capital crime and usually trials are held in court.
After the trial, he was executed.
After being found guilty, he was sentenced to execution. The sentence was carried out a few days later.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
He was tried and found guilty. The judge sentenced him to be hanged. During the night a mob gathered outside the jail and demanded that they be given the prisoner. In the morning his lifeless body hung from the gallows for all to see. There would be no appeal.