How important is Act 1 in establishing the main themes and underlying issues of the play?
There are many themes in act 1 which makes it clear to the audience what kind of genre this play is which is a tragedy. Through the themes that are in the first scene the audience will get an insight as to how the play will develop.
The first theme the audience come across is jealousy and this is portrayed through Iago, he is jealous that Othello had already "chosen my officer." This makes Iago very upset and shocked because he then starts comparing his attributes to Cassio's and describes him as a 'fair wife.' In this era men were the most dominant and were perceived as being better than women, so for a man to be called a 'wife' tells the audience that this man is weak, what this would do is make Iago seem a lot more powerful than Cassio and be better suited to be Othello's right hand man in his army.
In addition, Iago starts talking about the battles he has faced in 'Rhodes, at Cyprus, and on other grounds.' It is clear that Iago is trying to win the audience over by saying how brave he is and he uses the battlegrounds he has fought at to do so. By saying 'on other battle grounds' it makes the audience think as if Iago has been in so many battles that he cannot remember all of them; in this era, if you were in the army you were deemed a hero and the more battles you were in the more honour you would gain, so the fact that Iago has been in countless battles makes the audience start to think that maybe Othello made the wrong decision...