The differences between "The Tragedy of Macbeth" and "A Midsummer Night's Dream" are obvious in the theatre, but sometimes it's difficult to decide exactly what makes a play a tragedy or a comedy. There are less-obvious differences between the two play categories. There are some similarities between the two forms as well.
Study the characters in the two genres. Tragic characters tend to be simpler. This doesn't mean they are stupid, it means they approach problems in simple, straightforward ways. The world looks more black and white to the tragic character. The comic character is typically more complex and flexible in his attributes, approaching situations with more finesse. The world appears in shades of grey for comic characters. They also have to deal with more twists and turns in their plots than tragic characters do.
Notice another difference between the types of characters is their ability to engage emotionally. Lady Macbeth plots a murder, then goes insane and kills herself. In "A Midsummer Night's Dream," Titania is tricked by her lover Oberon into falling in love with a donkey. Once the spell is broken and Titania becomes herself again, she goes back to loving Oberon and lives happily ever after. While the two situations are vastly different, they exemplify the emotional engagement of the characters in each genre. Emotions drive tragic characters to commit acts of murder or suicide, while they drive comic characters to chase one another and play jokes.
Realise that tragic characters are concerned with the human soul, while comic characters tend to be more secular. Tragic characters act because of honour and revenge, while comic characters act because they want sex, food or fun.
Recognise that comedic characters are more forgiving than their tragic counterparts, and tend towards pacifism rather than militarism. Sir Toby Belch in Shakespeare's plays was once part of the military, but gave it up to drink all day. Othello, on the other hand, remains a warrior and upholds the honour of a soldier throughout his play.
See, too, that tragic characters, with few exceptions, use uncritical thinking. This means they do not question the social, political or religious order of things. They know that, by questioning, they could be severely punished in their world. Comic characters tend to be more critical thinkers, calling into question the social norms of their time. They point out the incongruities in the structure of their society.
Study the characters in each genre again, and you'll find that all characters are somehow flawed. The Greeks formed the idea of hubris, which is extreme haughtiness in a character. This is one flaw. Others include Lady Macbeth's ambition, Helena's love for Demetrius and Hamlet's inability to finish the deed of killing his uncle. All characters have flaws, which is why we love to watch them. They are like us.
Notice that in each genre, the story begins with a problem. The course of all plays takes characters through problems to a conclusion. Typically, in tragedy, the resolution is a sad one, ending in death, exile or the loss of a loved one. Comedies almost always end happily, with marriages, dancing and song; however, each genre always begins with a problem that must be solved.
Understand, too, how the narrative plots are similar. In "Oedipus Rex" and "A Midsummer Night's Dream," both plays begin with the same problem. There is a plague in the land, and it must be stopped. The former ends in Oedipus gouging out his own eyes because he realises he is the cause of the plague. Meanwhile, the latter ends in joy. The plague in "A Midsummer Night's Dream" was caused by two fairies, who fought, then reconciled their love in the end. The basic narrative plot points in each play are very similar.